Complete digital photography

Your digital camera makes color images using pretty much the same processMaxwell used in 1860: it combines three different black-and-white images to create a full-color final image.. The

C OMPLETE D IGITAL

THE CD-ROM THAT ACCOMPANIES THIS BOOK MAY BE USED ON A SINGLE PCONLY THE LICENSE DOES NOT PERMIT THE USE ON A NETWORK (OF ANYKIND) YOU FURTHER AGREE THAT THIS LICENSE GRANTS PERMISSION TO USETHE PRODUCTS CONTAINED HEREIN, BUT DOES NOT GIVE YOU RIGHT OF OWN-ERSHIP TO ANY OF THE CONTENT OR PRODUCT CONTAINED ON THIS CD-ROM.USE OF THIRD-PARTY SOFTWARE CONTAINED ON THIS CD-ROM IS LIMITED TOAND SUBJECT TO LICENSING TERMS FOR THE RESPECTIVE PRODUCTS.

CHARLES RIVER MEDIA, INC (“CRM”) AND/OR ANYONE WHO HAS BEEN VOLVED IN THE WRITING, CREATION, OR PRODUCTION OF THE ACCOMPANYINGCODE (“THE SOFTWARE”), OR THE THIRD-PARTY PRODUCTS CONTAINED ONTHIS CD-ROM, CANNOT AND DO NOT WARRANT THE PERFORMANCE OR RE-SULTS THAT MAY BE OBTAINED BY USING THE SOFTWARE THE AUTHOR ANDPUBLISHER HAVE USED THEIR BEST EFFORTS TO ENSURE THE ACCURACY ANDFUNCTIONALITY OF THE TEXTUAL MATERIAL AND PROGRAMS CONTAINEDHEREIN; WE, HOWEVER, MAKE NO WARRANTY OF THIS KIND, EXPRESS OR IM-PLIED, REGARDING THE PERFORMANCE OF THESE PROGRAMS THE SOFTWARE

IN-IS SOLD “AS IN-IS” WITHOUT WARRANTY (EXCEPT FOR DEFECTIVE MATERIALSUSED IN MANUFACTURING THE DISC OR DUE TO FAULTY WORKMANSHIP); THESOLE REMEDY IN THE EVENT OF A DEFECT IS EXPRESSLY LIMITED TO REPLACE-MENT OF THE DISC, AND ONLY AT THE DISCRETION OF CRM

THE AUTHOR, THE PUBLISHER, DEVELOPERS OF THIRD-PARTY SOFTWARE, ANDANYONE INVOLVED IN THE PRODUCTION AND MANUFACTURING OF THIS WORKSHALL NOT BE LIABLE FOR DAMAGES OF ANY KIND ARISING OUT OF THE USE OF(OR THE INABILITY TO USE) THE PROGRAMS, SOURCE CODE, OR TEXTUAL MA-TERIAL CONTAINED IN THIS PUBLICATION THIS INCLUDES, BUT IS NOT LIMITED

TO, LOSS OF REVENUE OR PROFIT, OR OTHER INCIDENTAL OR CONSEQUENTIALDAMAGES ARISING OUT OF THE USE OF THE PRODUCT

THE SOLE REMEDY IN THE EVENT OF A CLAIM OF ANY KIND IS EXPRESSLYLIMITED TO REPLACEMENT OF THE BOOK AND/OR CD-ROM, AND ONLY AT THEDISCRETION OF CRM

THE USE OF “IMPLIED WARRANTY” AND CERTAIN “EXCLUSIONS” VARY FROMSTATE TO STATE, AND MAY NOT APPLY TO THE PURCHASER OF THIS PRODUCT

No part of this publication may be reproduced in any way, stored in a retrieval system of any type, or transmitted by any means or media, electronic or mechanical, including, but not limited to, photocopy, recording, or scanning, without prior permission in writing from the publisher.

Cover Design: Tyler Creative

Cover Image: Ben Long

C HARLES R IVER M EDIA

This book is printed on acid-free paper.

Ben Long Complete Digital Photography

Long, Ben,

1967-Complete digital photography / Ben Long.

p cm.

Includes index.

ISBN 1-58450-520-6 (pbk with cd : alk paper)

1 Photography Digital techniques Handbooks, manuals, etc 2.

Digital cameras Handbooks, manuals, etc 3 Image processing Digital

techniques Handbooks, manuals, etc I Title

TR267.L66 2007

775 dc22

2007018885 Printed in the United States of America

Massachusetts 02210 CRM’s sole obligation to the purchaser is to replace the disc, based on defective materials or faulty workmanship, but not on the operation or functionality of the product.

eISBN-10: 1-58450-666-0

C ONTENTS

CH A P T E R 2 HO W A DI G I T A L CA M E R A WO R K S 11

CH A P T E R 3 BA S I C PH O T O G R A P H Y: A QU I C K PR I M E R 29

v

CH A P T E R 4 EV A L U A T I N G A N IM A G E 41

Contents vii

CH A P T E R 8 ME T E R I N G A N D EX P O S U R E 225

Tutorial 11.3 Correcting Barrel and Pincushion Distortion in Photoshop CS2/CS3 328

Tutorial 11.4 Correcting Perspective in Photoshop CS2/CS3 331Tutorial 11.5 Correcting Chromatic Aberration with Photoshop CS2/ CS3 333

CH A P T E R 13 BU I L D I N G YO U R ED I T I N G AR S E N A L 393

AP P E N D I X A GL O S S A R Y 539

25 51 81

0 0 7

122 137 135 137 33 160 158

0 0 2

0 0 0

20 48 99 122 137 33 127

0 0 0

0 0 0

0 0 12 16 40 130

124 191 206 214 216 214 214 216 224 232

In This Chapter

• Is This the Right Book for You?

• How This Book Is Organized

• What Counts as Digital Photography?

• The Best-Laid Plans

• What Software Do You Need?

• Digital 101: A Few Basic Ideas

It is something of a cliché now to write about the rapid advancement of digital

tech-nology—about how amazing it is to look back just a few years and see computersand digital cameras that had a fraction of the power, at a much higher price I knowthis, because I employ this cliché a lot It’s an easy hook to start an article or book, and

I used a variation of it in the introduction to the last edition of this book

And so, it is with some satisfaction that I find myself now able to write that, in

the two-and-a-half years since I wrote the last edition, there has not been a huge

leap in camera technology Oh sure, there’ve been some important advancementssince then: point-and-shoot cameras now routinely offer 8- or 10-megapixel sensors(the cameras I covered in the third edition topped out around 5 or 6); digital SLRsare cheaper than ever and offer more resolution; Canon now sells a full-frame SLRfor under $3000; new focusing technology makes it easier to get good shots from asmall camera; while affordable, quality SLR lens options are prevalent and plentiful.These advancements, though, are decidedly evolutionary, and the jump from an 8- to 10-megapixel camera is not nearly as significant as the jump from 4 to 6megapixels was Similarly, though the low-end price of a digital SLR has droppedfrom $800 to $600, this is not as dramatic as the change that happened between thesecond and third editions of this book, when bottom-end SLR prices dropped from

$2000 to $900

Nevertheless, despite the lack of dramatic technological advancement since thelast edition, I feel I can say with confidence that there has been more changebetween the third and fourth editions of this book, than with any other update

While other editions saw a greater leap in camera technology, this edition sees huge

changes in post-production and workflow

Those changes have made it easier to manage image glut, and have greatly fied the workflow of raw-format photography, opening up raw shooting to moreshooters, at more skill levels As such, a new chapter devoted exclusively to raw pro-cessing has been added, while I’ve also made extensive changes to all workflow discus-sions, and now provide coverage of new workflow applications like Apple’s Aperture™and Adobe®Photoshop®Lightroom™

simpli-But something else has happened over the last few years: as more and morepeople have gotten digital cameras, the average skill level has improved In addition

to improvements in camera technology, digital photography forums have facilitateddiscussions and learning, and photo sharing sites have made it possible for people toshow their work, and view the work of other shooters This combination of tech-nologies appears to be leading to a general improvement in the level of photographicskill many people have

I’ve noticed a change over the last few years of teaching photography For years,people were only interested in the technical craft They wanted to know what but-tons to push, how to use the various features of their camera, and how to performfancy edits in an image editor While that desire is still there, many students are nowcomfortable with the basic craft and are hungry to know more about what makes agood picture As such, this edition is the first one to spend time covering the “artis-tic” side of photography, with full discussions of composition, seeing, and shootingprocess

Chapter 1 Introduction 3

As with previous editions, you’ll still find discussions of all the technical ins andouts of shooting digital—from choosing a camera to exposing an image to processingand printing If you’re new to photography, you’ll learn the basic theory photogra-phers have studied for decades, and the latest tools and techniques made possible bythe shift to digital If you’re an experienced film photographer, you’ll see how youmight need to change your photographic process to adapt to digital shooting Fromwhite balancing to exposing, you’ll learn how to translate the knowledge you al-ready have into the digital realm

ISTHIS THERIGHTBOOK FORYOU?

This book is for photographers of all skill levels Whether you’re new to

photogra-phy or simply new to digital photographotogra-phy, this book provides the explanations and

answers you need to get the best images possible,

Using a film camera is a two-step process First, you shoot your pictures—withthe goal of getting as much color, contrast, and tone into your image as you canmanage Then, you develop and print the images in a way that takes maximum ad-vantage of that information Shooting digital photographs is no different As with afilm camera, you shoot your subject—after you, or your camera, first calculate theproper exposure Then, you digitally process and print your image

Although cameras might change, the physics of light remains the same nately!) Consequently, digital and film photographers share all the same concernsover apertures, shutter speeds, and metering techniques

(fortu-This book is divided into four broad sections Chapters 1 through 4 provide thebasic technical information you’ll need to understand all the topics covered later inthe book Chapters 5 and 6 help you select the camera, computer, and software thatare right for you Chapters 7 through 9 cover shooting, while the remaining chap-ters discuss post-production workflow, digital editing, and output

Because Complete Digital Photography is intended for users with a variety of rience levels, many terms have been defined in the Glossary rather than within the

expe-main text

No matter what your skill level is, you should find something of use in thisbook, as I have tried, within reason, to make it “complete.” In many cases, there hasnot been enough room to go into detail on some of the more obscure topics In thoseinstances, I have tried to at least let you know what the relevant questions and issuesare, so you can more quickly track down answers on your own

HOWTHISBOOKISORGANIZED

Chapter 2, “How a Digital Camera Works,” gets things rolling with a discussion ofhow a digital camera works Even if you’re not interested in all the nerdy details ofyour chosen gear, this chapter is important for all shooters because of its discussion

of the technical differences between shooting JPEG images, and shooting raw These

issues are unique to digital photography, so if you’re coming from the film world,you’ll want to learn more about this important consideration.

As mentioned earlier, whether you shoot digital or film, the process of exposing

an image remains the same If you’re new to photography, you’ll have an easiertime understanding the material in the book if you give a close read to Chapter 3,

“Basic Photography: A Quick Primer.” Even if you’ve been shooting for a while, ifyou have an automatic camera, there might be some basic exposure theory youhaven’t had to learn, simply because the camera has taken care of those issues foryou Learning the ins and outs of exposure, though, will provide you with moreartistic flexibility and choice, and our later shooting discussions assume an under-standing of the basic photographic theory presented in Chapter 3

Chapter 4, “Evaluating an Image,” walks you through the basics of “reading” thetechnical quality of the images you’ve shot Learning to recognize the technical dif-ferences between one image and another is handy for evaluating a camera whenyou’re ready to buy, and an essential skill when using image-editing software to cor-rect and improve an image This chapter has been re-written for clarity, and includesnew discussions of noise, detail, and sharpness

As with each previous edition, Chapter 5, “Choosing a Digital Camera,” has seenmany changes In addition to the usual updating to make the chapter relevant to thecurrent camera market, the chapter has been restructured to provide more clearlydelineated discussions of SLR and point-and-shoot cameras Many entirely new sec-tions have been added, including a discussion of external flashes, sensor cleaning,choosing a tripod, and much more

As in the previous edition, the book’s image-editing examples and tutorials arebuilt largely around Adobe Photoshop Although Photoshop is the premiere image-editing application, there are plenty of other good editors out there, and in Chapter

6, “Building a Workstation,” you’ll learn how to pick a package that provides thetools typically used by digital photographers Fortunately, most image-editing pro-grams provide the same basic tools and interfaces, so you should have no troubleadapting the lessons described here to other editing packages Since the last edition,the software landscape has changed dramatically due to the arrival of Apple’s Aper-ture and Adobe Photoshop Lightroom Both of those products are discussed inChapter 6, and additional coverage is provided throughout the rest of the book

As in previous editions, Chapter 6 will help you choose a computer system andmonitor, and the new version includes a revised, simpler color management section Chapter 7, “Shooting,” will get you started with your camera by showing youthe basics of shooting This chapter has been completely rewritten, and now in-cludes in-depth discussions on seeing, composition, how to warm up, and muchmore You’ll find plenty of technical information as well In addition to learninghow to configure your camera’s settings for particular situations, you’ll learn thebasic details of framing and focusing and simple flash photography Finally, you’lllearn the rudiments of light metering

Chapter 8, “Metering and Exposure,” builds on Chapter 7’s basic concepts butdelves into the creative possibilities provided by your camera’s manual exposurefeatures You’ll learn about motion control, depth of field, contrast, detail, and all

Chapter 1 Introduction 5

the other imaging choices that are possible when you take control of your camera’smanual features Chapter 8 has also been completely restructured and rewritten and

now includes an extensive Shooting for Raw section that details the exposure

ap-proach you’ll want to take when shooting raw

Knowing the basics doesn’t do you any good if you tend to shoot in unusual vironments or like shooting particularly tricky subjects Chapter 9, “Special Shoot-ing,” helps you with all of the more specialized forms of shooting you might engage

en-in, from macro photography to black and white Also restructured and rewritten,Chapter 9 includes many new sections such as dedicated coverage of landscapeshooting, high dynamic range imaging, sensor cleaning, backcountry shooting, ex-ternal flash shooting, and more

Once you have a camera full of images, you’re ready to start editing Chapter 10,

“Workflow,” covers everything you need to know about your post-production flow process An entirely new chapter, you’ll learn how to outline a basic workflow,what the considerations are for the way you shoot and what you need to deliver,how to use metadata to improve your workflow, and how to use the latest workflowsoftware to ease your post-production life

work-A digital image editor is capable of extraordinary retouching and special effects.However, 90 percent of the chores you’ll perform with your editing program arecolor and tone corrections In Chapter 11, “Correcting Tone and Color,” you’ll learnabout the basic color correction steps you’ll take on most of your images Coveringmultiple image-editing applications, by the end of the chapter, you will know how

to perform the bulk of the corrections you’ll need to make, and have a solid dation for the more advanced correction techniques discussed later in the book.Readers of previous editions will find many new discussions and tutorials in thischapter, and several new technique examples

foun-If you’ve opted to shoot raw (and if you’re not sure if you should, don’t worry,Chapters 2 and 8 will help you make up your mind), then Chapter 12, “Raw Conver-sion,” will help you through all the raw processing you’ll need to perform once you’reback from your shoot You’ll learn how to get the most out of your raw converter ofchoice, how to best approach raw editing, and how to easily work raw processing intoyour workflow

With basic correction skills under your belt, you’re ready to move on to Chapter

13, “Building Your Editing Arsenal,” where you will expand your repertoire with

a study of masking, cloning, making complex selections, and some basic layeringtechniques Chapter 13 includes many new masking and selecting techniques, andseveral new examples of blending and compositing tricks

Chapter 14, “Imaging Tactics,” kicks up your image-editing skills several notcheswith discussions of many advanced masking techniques Many new sections havebeen added in this edition, including grayscale conversion, retouching, adaptiveshadow/highlight adjustment, distortion correction, and more

Chapter 15, “Special Effects,” rounds out your post-production knowledge withdetailed descriptions of how to simulate shallow depth of field, stitch panoramas,

process high dynamic range images, add texture and grain, and more Finally, withyour images edited and corrected, you’re ready to output Chapter 16, “Output,” cov-

ers everything you need to know to get your images out of your computer, whether

through a printer, web page, or email Resizing, sharpening, color management, andprinter selection are all covered in detail in this heavily rewritten chapter

In addition to the tutorials from the first edition, four video tutorials are cluded on the CD-ROM that accompanies this book In these QuickTime® movies,you can watch “over my shoulder” as I perform a number of imaging and adjust-ment techniques Because many image correction processes are “interactive” ratherthan strictly procedural, these movies give you the opportunity to learn techniquesthat cannot be explained in a written, step-by-step form

in-The tutorial movies are included in the Tutorials folder on the companion ROM You can watch them in any order, but it would be best to view them whenthey are called out in the text, since some of the techniques assume you have alreadylearned certain concepts

CD-WHATCOUNTS ASDIGITALPHOTOGRAPHY?

Many people use the term digital photography to cover any photographic process that

involves correcting and editing a photograph digitally By this definition, shootingwith your film camera, scanning your film or prints, editing them on your computer,and then printing them on a desktop printer counts as digital photography

In this book, digital photographs are photographs taken with a digital camera.This is an important distinction because shooting with a film camera and shootingwith a digital camera involve different concerns and practices, and scanning is a sci-ence unto itself The simple fact is that there are many things you must do differ-ently with a digital camera But don’t worry—this book covers them all!

THEBEST-LAIDPLANS

Although much effort has gone into ensuring accuracy in this book, mistakes dosometimes happen An updated list of errata and corrections can be found at

www.completedigitalphotography.com/errata A quick check of this page will fill you in

on any problems that have been found in the book since its publication You can

report any mistakes by sending an email to oops@completedigitalphotography.com.

WHATSOFTWAREDOYOUNEED?

In Chapter 6, we’ll cover a number of guidelines for selecting various kinds ofphotography-related software, from image editors to image catalogers, and more.There are many choices when it comes to digital imaging programs, and which ap-plications you use will affect certain aspects of your post-production workflow, andyour editing options

ON THE CD

ON THE CD

Chapter 1 Introduction 7

Overall, this book attempts to be software agnostic This is not too difficult thesedays, as most image editing interfaces have been standardized, so it’s very easy totranslate a discussion built around one application to many other applications Nev-ertheless, Adobe Photoshop still rules the roost when it comes to image editing, andthe bulk of this book’s tutorials and discussions have been built around Photoshop.However, many of these sections also include additional information with guidelinesfor performing the same tasks in Apple’s Aperture, Adobe Photoshop Lightroom,and Nikon™’s Capture NX™ Where possible, I’ve tried to separate out each discus-sion, so you only need to deal with the relevant information The practical upshot isthat, no matter what software you’re using, you should be able to easily followalong with the examples, theories, and discussions provided herein

DIGITAL101: A FEWBASICIDEAS

In the old days, photographers made their own photographic papers, films, and

emulsions Whether it was to achieve a particular style or texture or to gain more

control over their printing processes, photographers such as Ansel Adams, AlfredStieglitz, and Eduard Steichen had to know a good deal about chemistry to createtheir prints Similarly, to really understand how to get the most out of your digitalcamera, it’s important to understand some of the technology behind it Chapter 2covers the details of how a digital camera works, but before you dive into that, you

need to have a basic understanding of the digitizing process.

The “real” world in which we live is an analog world Light and sound come to

us as continuous analog waves our senses interpret Unfortunately, it’s very difficult

to invent a technology that can accurately record a continuous analog wave For ample, you can cut a continuous wave into a vinyl record, but because of the limita-tions of this storage process, the resulting recording is often noisy and scratchy andunable to capture a full range of sound

ex-Storing a series of numbers, on the other hand, is much simpler You can carvethem in stone, write them on paper, burn them to a CD-ROM, or, in the case of dig-ital cameras, record them to small electronic memory chips Moreover, no matterhow you store them, as long as you don’t make any mistakes when recording them,you’ll suffer no loss of data or quality as you move those numbers from place toplace Therefore, if you can find a way to represent something in the real world as aseries of numbers, you can very easily store those numbers using your chosenrecording medium

The process of converting something into numbers (or digits) is called digitizing.

The first step in digitizing is to divide your subject into distinct units In the case of

an image, this is a simple process of dividing your image into a grid of picture elements

or pixels How fine your grid is (that is, what resolution it has) varies depending on the

sophistication of your equipment

Next, each pixel in the grid is analyzed to determine its content, a process called

sampling Each sample is measured to determine how “full” it is; that is, a

correspond-ing numeric value is assigned that represents that pixel’s contents, a process called

quantizing Finally, these numeric values are stored on some type of storage medium.

Figure 1.1 is a simple image composed entirely of black and white pixels As youcan see, it’s very easy to assign a 1 or a 0 to each pixel to represent the image Because

it takes only a single bit to represent each pixel, this image is called a 1-bit image.

FIGURE 1.1 Each pixel in this 1-bit image is represented by a 1 or a 0.

=

1

1 1 1

1 1 1

1 1

1 11 1 1 1 1 1 1 1 1

1 1 1 1

1

1 1 1 1

1

1

1 1 1

1

1 1 1 1

0 0 0 0

0 0 0 0

0 0

0 0

0 0 0

0 0 0 0 0 0 0

0 0 0 0 0

0 0 0

0 0

0 0 0

0 0 0 0 0 0 0 0

0 0 0 0

0 0 0

0 0 0 0 0

0 0 0 0

0 0 0

0 0

0 0 0

0 0 0 0

0 0 0

0 0 0

0 0

0 0

0 0 0

0 0 0

0

0 0 0 0 0 0 0

0 00 0 0 0 0 0

0 0 0 0

0 0

0 0 0

0 0 0

0 0

0 0 0 00 0 0 0

0 0 0

0 0 0 0

0 0 0

0 0 0

0 0 0

0

0 0 0

0 0 0 0

0

0 0 0 0 0 0

0 0 0

0 0

0 1 1 1

1111 1 11 11

1 1 1

1 1

1 1

1 1

1 1

1 11 1

1

1 1 1 1 1

1

1 1 1 1

1 1 1 1 1

1 1

1 1 1

0 0 0

0 0 0 0

0 0

0

0 0 0 0

0

0 0

0 0 0

0 0 0 0

0 0 0 0 0

0

0

0 0 0

0 0 0

0 0 0 0

0 0

0

0 0 0 0

0

0 0

0 0 0

0 0 0 0 0

0

0 0

0 0 0

0 0 0 0

0 0 0 0

0

0 0

0

0 0 0

0 0 0

0 00 1

FIGURE 1.2 By storing bigger numbers for each pixel, we can store much more than simple black and white dots With the ability to use gray pixels, the image looks much more realistic

25 51 81

0 0 7

122 137 135 137 33 160 158

0 0 2

0 0 0

20 48 99 122 137 33 127

0 0 0

0 0 0

0 0 12 16 40 130

124 191 206 214 216 214 214 216 224 232

In the example shown in Figure 1.1, our individual samples can be only 0 or 1

(that is, they have a very small dynamic range) because we are storing only one bit of

information per pixel If we want to record more than simple black and white ages, we need to be able to specify more colors—that is, we need to have more

im-choices than just 0 or 1 By going to a higher color depth (sometimes called bit

depth)—let’s say 8 bits, which allows for 256 different values—we can record more

information, as seen in Figure 1.2

To sum up, two of the factors that determine the quality of a digitizing processare resolution (the size and number of your individual pixels) and dynamic range(how big a range of color choices you have for each pixel)

Many other factors affect the quality of a digitized image, from your camera’slens to its compression software Before exploring these questions, though, let’s look

at how your camera manages all of this sampling, quantizing, and storage standing how your camera perceives, captures, and stores color information willmake certain types of editing operations easier later on

• Something Old, Something New

• A Little Color Theory

• Meanwhile, Back in the Real World

While it doesn’t take a lot of know-how to operate a fully automatic camera,

serious photography—whether at the hobbyist or professional level—hasalways required a tremendous amount of craftsmanship and technicalunderstanding In “wet darkroom” work, everything from paper type to chemicalmixtures to the amount of agitation applied to a particular chemical has a bearing onthe quality of your final print As such, skilled film photographers have to have an in-depth understanding of the nature of their papers, chemicals, and equipment.Digital photography is no different To fully exploit your camera’s capabilities—and to be able to effectively edit and improve your images—you must understandsome of the fundamental principles and technologies behind digital imaging

As mentioned in Chapter 1, “Introduction,” the only real difference between adigital camera and a film camera is that a digital camera does not use film to record

an image However, this one fundamental difference affects all of the other systems

on the camera, from the lens to the light meter Consequently, knowing some of thetechnical details of how a digital camera works will help you select the right cameraand help you better understand how to make certain decisions when shooting

SOMETHINGOLD, SOMETHINGNEW

Just like a film camera, your digital camera records an image by using a lens to focus

light through an aperture and a shutter and then onto a focal plane By opening or

closing the aperture and by changing the amount of time the shutter is open, thephotographer can control how the focal plane is exposed As we’ll see later, exposurecontrol allows the photographer to change the degree to which the camera “freezes”motion, how well the film records contrast and color saturation, and which parts ofthe image are in focus

While a film camera has a piece of film sitting on the focal plane, in a digital

camera, an image sensor is mounted on the focal plane An image sensor is a special

type of silicon chip that is light sensitive Currently, there are two major types of

image sensors available: the charge-coupled device (CCD), and the Complementary Metal

Oxide Semiconductor (CMOS) CCDs are more popular than CMOS chips, but the role

of both is the same When you take a picture, the light falling on the image sensor is

sampled, converted into electrical signals After the image sensor is exposed, these

signals are boosted by an amplifier and sent to an analog-to-digital converter thatturns the signals into digits These digits are then sent to an onboard computer forprocessing Once the computer has calculated the final image, the new image data isstored on a memory card (See Figure 2.1.)

Foveon

There’s a third type of image sensor made by a company called Foveon We’ll discuss it in Chapter 5, “Choosing a Digital Camera.” CMOS and CCDs remain the dominant image sen- sor technology.

Chapter 2 How a Digital Camera Works 13

Although the mechanics are simple to explain, to really understand how a tal camera functions, you must know a little color theory

digi-A LITTLECOLORTHEORY

In 1869, James Clerk Maxwell asked photographer Thomas Sutton (the inventor ofthe SLR camera) to take three black-and-white photographs of a tartan ribbon.Maxwell wanted to test a theory he had about a possible method for creating colorphotographs He asked Sutton to place a different filter over the camera for eachshot: first, a red filter, then green, and then blue After the film was developed,Maxwell projected all three black-and-white pictures onto a screen using three pro-jectors fitted with the same filters that were used to shoot the photos When the im-ages were projected directly on top of each other, the images combined and Maxellhad the world’s first color photo

This process was hardly convenient Unfortunately, it took another 30 years toturn Maxwell’s discovery into a commercially viable product This happened in

1903, when the Lumière brothers used red, green, and blue dyes to color grains ofstarch that could be applied to glass plates to create color images They called theirprocess Autochrome, and it was the first successful color printing process

In grammar school, you probably learned that you could mix primary colors gether to create other colors Painters have used this technique for centuries, ofcourse, but what Maxwell demonstrated is that, although you can mix paints to-gether to create darker colors, light mixes together to create lighter colors Or, to use

to-some jargon, paint mixes in a subtractive process (as you mix, you subtract color to create black), whereas light mixes in an additive process (as you mix, you add color

to create white) Note that Maxwell did not discover light’s additive properties—Newton had done similar experiments long before—but Maxwell was the first toapply the properties to photography

FIGURE 2.1 Light passes into a digital camera, just as it would in a film

camera However, instead of hitting a piece of film, it is digitized by a computer chip and passed to an onboard computer to create an image.

Light

A to D Converter Onboard Amplifier

A to D Converter Onboard Amplifier

A to D Converter 0110100 ComputerOnboard

Amplifier

Figure 2.2 shows a simple example of how the three additive primary colors oflight can be combined to create other colors.

FIGURE 2.2 Red, green, and blue—the three additive primary colors of

light—can be mixed together to create other colors As you combine them, the resulting color gets lighter, eventually becoming white Note also that where the colors overlap they create the secondary primary colors—cyan, magenta, and yellow These are the primary colors of ink.

Your digital camera makes color images using pretty much the same processMaxwell used in 1860: it combines three different black-and-white images to create

a full-color final image

The image shown in Figure 2.3 is called an RGB image because it uses red, green, and blue channels to create a color image

FIGURE 2.3 In a digital image, three separate red, green, and

blue channels are combined to create a final, full-color picture.

RGB IMAGE BLUE CHANNEL GREEN CHANNEL RED CHANNEL

Chapter 2 How a Digital Camera Works 15

This color theory is not just a trivial history lesson Understanding that your fullcolor images are composed of separate channels will come in very handy later, whenyou start editing Very often, you’ll correct color casts and adjust your images byviewing and manipulating individual color channels

You Say “Black and White,” I Say “Grayscale”

Although film photographers use the term black-and-white to denote an image that lacks color, in the digital world it’s better to use the term grayscale As we saw in Figure 1.1, your computer can create an image that is composed of only black and white pixels Consequently, it’s sometimes important to distinguish between an image that is made up of black and white pixels, and one that is made up of pixels of varying shades of gray.

In the century and a half since Maxwell’s discovery, many other ways of

repre-senting color have been discovered For example, another model called L*A*B color

(also known as Lab color) uses one channel for lightness information, another channelfor greenness or redness, and a third channel for blueness or yellowness In addition,

there is the cyan, magenta, yellow, and black (CMYK) model that printers use.

Each of these approaches is called a color model, and each model has a particular

gamut, or range, of colors it can display Some gamuts are more appropriate to

cer-tain tasks than others are, and all are smaller than the range of colors your eye canperceive

We’ll deal more with gamuts and color models in later chapters For now, it’simportant to understand that digital photos are made up of separate red, green, andblue channels that combine to create a color image

HOW ANIMAGESENSORWORKS

George Smith and Willard Boyle were two engineers employed by Bell Labs Thestory goes that one day in late October, the two men spent about an hour sketchingout an idea for a new type of semiconductor that could be used for computer mem-ory and for the creation of a solid-state, tubeless video camera The year was 1969,and in that hour, the two men invented the CCD

Roughly a year later, Bell Labs created a solid-state video camera using Smithand Boyle’s new chip Although their original intention was to build a simple cam-era that could be used in a video-telephone device, they soon built a camera thatwas good enough for broadcast television

Since then, CCDs have been used in everything from cameras to fax machines.Because video cameras don’t require a lot of resolution (only half a million pixels orso), the CCD worked great for creating video-quality images For printing pictures,though, you need much higher resolution—millions and millions of pixels Conse-quently, it wasn’t until recently that CCDs could be manufactured with enough res-olution to compete with photographic film

To a degree, you have Albert Einstein to thank for your digital camera, because

he was the first to explore the photoelectric effect It is because of the photoelectric

ef-fect that some metals release electrons when exposed to light (Einstein actuallywon the 1921 Nobel Prize in physics for his work on the photoelectric effect, not forhis work on relativity or gravity, as one might expect.)

The image sensor in your digital camera is a silicon chip that is covered with a

grid of small electrodes called photosites, one for each pixel (See Figure 2.4.)

Before you can shoot a picture, your camera charges the surface of the CCDwith electrons Thanks to the photoelectric effect, when light strikes a particularphotosite, the metal in that site releases some of its electrons Because each photo-site is bounded by nonconducting metal, the electrons remain trapped In this way,each photosite is like a very shallow well, storing up more and more electrons asmore and more photons hit After exposing the CCD to light, your camera simplyhas to measure the voltage at each site to determine how many electrons are thereand, thus, how much light hits that particular site (As was discussed in Chapter 1,

this process is called sampling.) This measurement is then converted into a number

by an analog-to-digital converter

CCD VS CMOS

Ninety to 95 percent of the digital cameras you’ll look at will use CCD image sensors The rest will use a CMOS chip of some kind What’s the difference? Because much more re- search has been put into CCD technology, it’s more prevalent CMOS chips, though, are ac- tually much cheaper to produce than the difficult-to-make CCDs found in most cameras CMOS chips also consume much less power than does a typical CCD, making for longer camera battery life and fewer overheating problems CMOS also offers the promise of in- tegrating more functions onto one chip, thereby enabling manufacturers to reduce the number of chips in their cameras For example, image capture and processing could both

be performed on one CMOS chip, further reducing the price of a camera.

Although CMOS used to have a reputation for producing rough images with inferior color, Canon ® ’s excellent EOS series of digital SLRs have shown that CMOS can be a viable alternative to CCDs.

Both chips register light in the same way, and for the sake of this discussion, the two technologies are interchangeable In the end, image sensor choice is irrelevant as long as the camera delivers an image quality you like.

Chapter 2 How a Digital Camera Works 17

Most cameras use either a 12-bit or 14-bit analog-to-digital converter That is,the electrical charge from each photosite is converted into a 12- or 14-bit number Inthe case of a 12-bit converter, this produces a number between 0 and 4,096; with a14-bit converter, you get a number between 0 and 16,384 Note that an analog-to-digital converter with a higher bit depth doesn’t give your CCD a bigger dynamicrange The brightest and darkest colors it can represent remain the same, but the

extra bit depth does mean that the camera will produce finer gradations within that

dynamic range As you’ll see later, how many bits get used in your final image pends on the format in which you save the image

de-The term charge-coupled device (CCD) is derived from the way the camera reads the

charges of the individual photosites After exposing the CCD, the charges on the first

row of photosites are transferred to a read-out register where they are amplified and

then sent to an analog-to-digital converter Each row of charges is electrically coupled

to the next row so that, after one row has been read and deleted, all of the other rowsmove down to fill the now empty space (See Figure 2.5.)

FIGURE 2.4 The sensor from a Nikon D70 has

an imaging area of 23.7 mm by 15.6 mm.

FIGURE 2.5 Rows of photosites on a CCD are coupled together As the bottom

row of photosites is read off the bottom of the CCD, all of the rows above it shift down This is the coupled in charge-coupled device.

Coupled rows shift down

Current row shifts into the readout register

A-D Converter

To processor

Amplifier

After all the rows of photosites have been read, the CCD is recharged with trons and is ready to shoot another image.

elec-Photosites are sensitive only to how much light they receive; they know nothingabout color As you’ve probably already guessed, to see color your camera needs toperform some type of RGB filtering similar to what James Maxwell did There are a

number of ways to perform this filtering, but the most common is through a single

array system, sometimes referred to as a striped array.

Arrays

Consider the images in Figure 2.6

If asked to fill in any “missing” pixels in Figure 2.6a, you’d probably say, “Whatare you talking about?” If asked to fill in any “missing” pixels in Figure 2.6b, though,you probably would have no trouble creating the image in Figure 2.7

FIGURE 2.6 Although you have no idea what pixels belong in Figure 2.6a, you can probably hazard a

guess as to what the missing pixels are in Figure 2.6b.

You would know which pixels you needed to fill in based on the other pixels

that were already in the image In other words, you would have interpolated the new

pixels based on the existing information You might have encountered interpolation

if you’ve ever resized a photograph using an image-editing program such as shop To resize an image from 2,048 × 1,536 pixels to 4,096 × 3,072 pixels, yourimage editor has to perform many calculations to determine what color all of thosenew pixels should be (Obviously, in this example, your ability to interpolate isbased on your ability to recognize a familiar icon—the happy face An image editorknows nothing about the content of an image, of course, and must interpolate bycarefully examining each of the pixels in an image to determine what the colors ofany new additional pixels should be.)

Photo-Chapter 2 How a Digital Camera Works 19

A typical digital camera uses a form of interpolation to create a color image As

we saw in the previous section, the image sensor in your camera is able to create agrayscale image of your subject by measuring the amount of light that strikes eachpart of the image sensor To shoot color, your camera performs a variation of thesame type of RGB filtering Maxwell used in 1869 Each photosite on your camera’simage sensor is covered by a filter—red, green, or blue This combination of filters is

called a color filter array, and most image sensors use a filter pattern like the one shown in Figure 2.8, called the Bayer Pattern.

FIGURE 2.7 If asked to fill in the “missing”

pixels in Figure 2.6b, you would probably come up with an image something like this

FIGURE 2.8 To see color, alternating pixels on an image sensor are

covered with a different colored filter The color filter array shown

here is called the Bayer Pattern.

G G G G G G G G

G G G G G G G G G

G G G G G G G G

G G G G G G G G G

G G G G G G G G

G G G G G G G G G

G G G G G G G G G G G G G G G G G

G G G G G G G G

G G G G G G G G G

G G G G G G G G

G G G G G G G G G

G G G G G G G G G G G G G G G G

With these filters, the image sensor can produce separate, incomplete red,green, and blue images The images are incomplete because the red image, for ex-ample, is missing all of the pixels that were covered with a blue filter, whereas theblue filter is missing all of the pixels that were covered with a red filter Both the redand blue images are missing the vast number of green-filtered pixels.

A sophisticated interpolation method is used to create a complete color image Just

as you used the partial pixel information in Figure 2.6b to calculate the missing pixels,your digital camera can calculate the color of any given pixel by analyzing all of the ad-jacent pixels For example, if you look at a particular pixel and see that the pixel to theimmediate left of it is a bright red pixel, the pixel to the right is a bright blue pixel, andthe pixels above and below are bright green, then the pixel in question is probablywhite Why? As Maxwell showed, if you mix red, green, and blue light together, youget white light (By the way, if you’re wondering why there are so many more greenpixels than red or blue pixels, it’s because the eye is most sensitive to green Conse-quently, it’s better to have as much green information as possible.)

This process of interpolating is called demosaicing, and different vendors employ

different approaches to the demosaicing process For example, many cameras look

at only immediately adjacent pixels, but Hewlett-Packard cameras analyze a region

up to 9 × 9 pixels The Fuji®SuperCCD eschews the grid pattern of square photosites

in favor of octagonal photosites arranged in a honeycomb pattern Such a schemerequires even more demosaicing to produce rectangular image pixels, but Fujiclaims this process yields a higher resolution Differences in demosaicing algorithmsare one factor that makes some cameras yield better color than others

Some cameras use a different type of color filter array Canon, for example, oftenuses cyan, yellow, green, and magenta filters on the photosites of their image sensors.Because it takes fewer layers of dye to create cyan, yellow, green, and magenta filtersthan it does to create red, green, and blue filters, more light gets through the CYGMfilter to the sensor (Cyan, yellow, and magenta are the primary colors of ink, andtherefore don’t need to be mixed to create the color filters; hence, they aren’t asthick.) More light means a better signal-to-noise ratio, which produces images withless noise

As another example, Sony sometimes uses red, green, blue, and emerald filters.They claim these filters give a wider color gamut, yielding images with more accu-rate color Dissenters argue that this approach results in bright areas of the imagehaving a cyan color cast

Image sensors are often very small, sometimes as small as 1/4 or 1/2 inch (6 or

12 mm, respectively) By comparison, a single frame of 35 mm film is 36 × 23.3 mm.(See Figure 2.9.) The fact that image sensors can be so small is the main reason whydigital cameras can be so tiny

By packing more and more photosites onto an image sensor, chipmakers can crease the sensor’s resolution However, there is a price to pay for this To pack morephotosites onto the surface of the chip, the individual sites have to be made muchsmaller As each site gets smaller, its capability to collect light is compromised be-cause it simply doesn’t have as much physical space to catch passing photons This

in-limitation results in a chip with a poor signal-to-noise ratio; that is, the amount of

Chapter 2 How a Digital Camera Works 21

good data the chip is collecting—the signal—is muddied by the amount of noise—noise from the camera’s electronics, noise from other nearby electrical sources, noisefrom cosmic rays raining down from space—the chip is collecting

In your final image, this signal-to-noise confusion can manifest as grainy terns in your image—visible noise like what you see on a staticky TV channel—orother annoying artifacts (See the noise example in Figure 4.2.)

pat-To improve the light-collecting capability of tiny photosites, some chipmakers sition tiny microlenses over each photosite These lenses focus the light more tightlyinto the photosite in an effort to improve the signal-to-noise ratio However, theselenses can cause problems of their own in the form of artifacts in your final image.Image sensors suffer from another problem that film lacks If too much light hits

po-a ppo-articulpo-ar photosite, it cpo-an spill over into po-adjpo-acent photosites If the cpo-amerpo-a’s

soft-ware isn’t smart enough to recognize that this has happened, you will see a blooming

artifact—smearing colors or flared highlights—in your final image Blooming ismore prevalent in a physically smaller image sensor with higher resolution becausethe photosites are packed more tightly together This problem is not insurmount-able, and even if your image does suffer from blooming problems from time to time,these artifacts won’t necessarily be visible in your final prints

As you might expect, interpolating the color in a camera with millions of pixels

on its image sensor requires a lot of processing power Such power (and the memoryneeded to support it) is one reason why digital cameras have stayed so pricey—lots

of fancy chips are necessary to make a digital camera

Extra Pixels

Not all of the photosites in an image sensor are used for recording your image Some are used

to assess the black levels in your image; others are used for determining white balance Finally, some pixels are masked away altogether For example, if the sensor has a square array of pixels but your camera manufacturer wants to create a camera that shoots rectangular images, they will mask out some of the pixels on the edge of the sensor to get the picture shape they want.

FIGURE 2.9 Most CCDs are very small, particularly when

compared to the size of 35 mm film

1 8

35mm Film - 36mm x 23.3mm APS Film - 30.2mm x 16.7mm

EOS 10D/Rebel CMOS 22.7mm x 15.1mm

HOWYOURCAMERAMAKES ANIMAGE

The preceding description might sound complicated In reality, though, the process

an imaging chip employs to capture an image is even more complex

First, the light coming through the lens is passed through several filters Theseinclude an infrared filter (some cameras use a very slight infrared filter, makingthem ideal for infrared photography, as we’ll see in Chapter 7, “Shooting”) and alow pass filter, both of which help ensure better color and fewer artifacts The image

is also blurred ever so slightly

This last fact often causes some photographers to turn pale, particularly thosewho have just spent lots of money on very sharp lenses The blurring is there to help

in the demosaicing process Even though an individual pixel is extremely small, it is

theoretically possible for a single pixel, or line of pixels, to be a separate color fromits surrounding pixels This could confuse a demosaicing algorithm, so a slight blur isapplied so the colors of individual pixels are smeared ever so slightly into theirneighbors

After you take a picture, the data is read off the image sensor, amplified, passedthrough an analog-to-digital converter (Figure 2.5), and then passed to your cam-era’s on-board processor There, it is demosaiced to produce a color image However,straight demosaicing does not produce an accurate, attractive color image A littlemore calculation is required

Colorimetric Adjustment

First, the image goes through a “colorimetric adjustment.” During demosaicing, thecamera’s processor knows that a particular pixel was red or green or blue (or what-ever colors are used in the color filter array), but it doesn’t know which preciseshades of those primary colors were used in the filters So, the color in the image isskewed slightly to compensate for the specific color qualities of the color filter array.Your camera now has a more accurate idea of the colors of each pixel in yourimage For example, it might know that one particular pixel is 100 percent of a par-ticular shade of chartreuse But what exactly does 100 percent mean? For this num-ber to be meaningful, some boundaries need to be defined, and so your image is

mapped into a color space.

Color Space Conversion

A color space is simply a mathematical model that can be used to represent colors.Some color spaces are larger than others, and some might allow for more variation

in a particular color—reds, for example Without a color space, your image would be

a fairly meaningless array of numbers Your camera probably provides a choice oftwo different color spaces, sRGB and Adobe RGB You’ll learn much more aboutcolor spaces choices in Chapter 7

Chapter 2 How a Digital Camera Works 23

Gamma Correction

In an imaging chip, when twice as much light hits a single pixel, twice as much

volt-age is produced In other words, the response of the pixels to light is linear—increase

the light and you get a linear increase in voltage Your eyes don’t work this way.When you increase brightness, your eyes register a logarithmic increase, rather than

a linear increase So, a doubling of light does not make you perceive a scene as beingtwice as bright

The practical upshot of all of this is that your eyes are able to see a lot of reallyfine detail in shadow and highlight areas As an example, consider the upper image

in Figure 2.10 This grayscale ramp goes from black to white in a linear fashion This

is how your camera sees a change in brightness

FIGURE 2.10 The upper ramp makes a linear progression from

black to white—this is how your digital camera’s sensor sees the world The lower ramp makes a nonlinear progression.

This is the type of light response your eyes have.

The lower image goes from black to white in a nonlinear fashion Notice that there

is much more variation in the lower fourth and upper fourth of the nonlinear ramp Inother words, the shadows and highlights have been expanded to show more variation Because of their nonlinear nature, your eyes tend to expand the shadow andhighlight areas, registering more subtle changes, and allowing you to see more de-tail As we’ll discover in many places in this book, your eyes are extremely sensitive

to subtle changes in contrast—much more than they are to changes in color This isbecause the light-sensitive portion of your eye is composed mostly of luminance-sensitive rods, while only a tiny part is composed of color-sensitive cones

So, to get accurate brightness values—that is, to expand the highlights and ows so they appear more like what your eye is used to—your camera applies a

shad-mathematical curve to all its brightness values This is called a gamma curve, times called gamma correction (Figure 2.11)

some-White Balance and Image Processing

Next, the camera adjusts your image according to your white balance setting Yourcamera might also allow you to select several other adjustments, from contrast tobrightness to color saturation These adjustments are just like the ones you might

apply in an image editor and are usually applied after white balancing We’ll discusswhite balance and in-camera adjustments in Chapter 7.

Sharpening and Noise Reduction

Finally, many cameras perform some type of noise reduction algorithm to reduce

unwanted noise in your image, and almost all cameras perform some type of

sharp-ening We’ll discuss noise more in Chapter 4, “Evaluating an Image,” and you can

see examples of noisy images in Figures 4.2 and 4.3

JPEG Compression and Saving

Your image is now processed and ready to save At this point, JPEG compression will

be applied, and then your finished, compressed image will be written to your era’s media card

cam-All this processing takes place as soon as your image has been shot, and as youmight imagine, it can take a while to perform all these calculations I mean “take awhile” in computer terms, of course, but even though your camera’s processor isvery speedy, it can still be overwhelmed with image processing To counter this,most cameras include an extra memory buffer that allows them to cache a few im-ages for processing, freeing up the camera for immediate shooting

FIGURE 2.11 The upper image shows this picture before gamma correction Note

the difference in overall contrast As with the gray ramp in Figure 2.10, the lower, gamma-corrected image has darker shadows and more varied highlights

Chapter 2 How a Digital Camera Works 25

The procedure described here is what is required to turn the image data yourcamera captures into a final image When you shoot in JPEG mode, all of that pro-cessing happens in your camera’s on-board computer However, your camera mayoffer an additional option

Raw Images

If your camera provides a raw mode, you have the option of deferring all the stepsdescribed in the previous section In raw mode, your camera reads the data off thecamera sensor, and writes it to your camera’s storage card No processing is per-formed—not even demosaicing Because these files are larger than JPEG files, they’llconsume more space on your card and take longer to write (potentially slowingdown your burst shooting)

To turn a raw file into a usable image, you must run it through a raw converter, a

program you run on your desktop computer, which performs all the same steps yourcamera would have taken if you had shot the image in JPEG mode However, rawshooting offers several advantages over JPEG shooting:

• Because you’re running the conversion process on your computer, you can takecontrol of many of the conversion steps This extra control will often let youproduce better images

• Your computer is not rushing to try to prepare to take a picture, so it can usesome more complex algorithms for its raw conversions These algorithms canoften yield better results

• When you shoot in raw mode, you can set the white balance of your imageafter-the-fact For shooting in tricky lighting situations, and for certain types ofcolor corrections, this feature affords you a level of flexibility you can’t get anyother way

• While your camera may capture 10–14 bits of color per pixel, JPEG images onlyallow 8 bits of color per pixel So, when your camera converts to JPEG mode, itthrows out a lot of the color information it captured The resulting image willhave less “room” for editing In other words, dramatic edits will more quicklyyield artifacts With a raw file, you can choose to output your image as a 16-bitfile Your raw converter won’t add any additional data, but it will be able to pre-

serve all the color information it captured, allowing you more editing latitude.

• As discussed earlier, your camera probably lets you dial in certain adjustments—contrast, tone, saturation, sharpening Once these adjustments are applied, it’svery difficult to remove them So, if your camera is a little too aggressive with itsadjustments, you may face some difficult editing What’s more, these adjust-ments “use up” some of your editing latitude In raw mode, since you’re in con-trol of all processing, you won’t ever face an “already over-edited” image

• Raw processing algorithms are constantly improving As new raw processorscome out, you can return to your old raw files and reprocess them If you have

a particularly difficult image that you’ve never been able to get quite right, anew raw processor might one day allow you to get the image you want

• Because raw images aren’t compressed, they never suffer from the JPEG facts that can plague compressed images.

arti-We’ll have much more to say about raw files in Chapters 8, “Metering and posure” and Chapter 12, “Raw Conversion.”

After your image has been processed, it’s ready to be stored on whatever storagemedium is provided by your camera While there are many different storage optionsfor camera makers to choose from, they all have one thing in common: they’re finite.Consequently, to make the most of the available storage, cameras compress their im-

ages, usually using a type of compression called JPEG.

Created by the Joint Photographic Experts Group, JPEG is a powerful algorithm

that can greatly reduce the size of a photo but at the cost of image quality

Conse-quently, JPEG is referred to as a lossy compression format.

When saving in JPEG mode, the camera first converts the image data from itsoriginal 12- or 14-bit format down to an 8-bit format, reducing the range of bright-ness levels from 4,096 or 16,384 all the way down to 256 Once the data is in 8-bitmode, the camera is ready to start compressing

Most cameras offer two forms of JPEG compression: a low-quality option thatvisibly degrades an image but offers compression ratios of 10 or 20:1, and a high-quality option that performs a good amount of compression—usually around 4:1—but without severely degrading your image Some cameras offer an even finer JPEGcompression that cuts file sizes while producing images that are indistinguishablefrom uncompressed originals In most cases, you’ll probably find that any artifactsintroduced by higher-quality JPEG compression are not visible in your final prints.JPEG compression works by exploiting the fact that human vision is more sensi-tive to changes in brightness than to changes in color To JPEG-compress an image,your camera first converts the image into a color space where each pixel is expressed

using a chrominance (color) value and a luminance (brightness) value.

Next, the chrominance values are analyzed in blocks of 8 × 8 pixels The color ineach 64-pixel area is averaged so that any slight (and hopefully imperceptible)

change in color is removed, a process known as quantization Note that because the

averaging is being performed only on the chrominance channel, all the luminance formation in the image—the information your eye is most sensitive to—is preserved.After quantization, a nonlossy compression algorithm is applied to the entire

in-image In the very simplest terms, a nonlossy compression scheme works something

like this: rather than encoding AAAAAABBBBBCCC, you simply encode 6A5B3C.After quantization, the chrominance information in your image will be more uni-form, so larger chunks of similar data will be available, meaning that this final com-pression step will be more effective

What does all this mean to your image? Figure 2.12 shows an image that hasbeen overcompressed As you can see, areas of flat color or smooth gradations haveturned into rectangular chunks, whereas contrast in areas of high detail has been

Chapter 2 How a Digital Camera Works 27

boosted too high Fortunately, most digital cameras offer much better compressionquality than what you see here

FIGURE 2.12 This image has been compressed far too much,

as can be seen from the nasty JPEG artifacting.

MEANWHILE, BACK IN THEREALWORLD

If the information in this chapter seems unnecessary, it’s probably because whenyou buy a film camera you don’t have to worry about imaging technology—it’s in-cluded in the film you use However, if you’re serious about photography, you prob-ably do spend some time considering the merits of different films And, just as youneed a little knowledge of film chemistry to assess the quality of a particular filmstock, the topics covered in this chapter will help you better test a particular camera Your camera is more than just an image sensor, of course, so in Chapter 5, you’lllearn about the other components and concerns you’ll need to weigh when choos-ing a camera

• Exposures: Apertures, Shutter Speeds, and ISO

• Mostly the Same