Photoshop cs5 cho nhiếp ảnh gia part 39 doc

the primary component of the levels dialog box is a histogram display shown earlier in Figure 6.3 that charts the distribution of tonal values within your image.. that some pixels in you

Tonal Adjustments with Levels

the levels adjustment provides good basic control over tonal adjustments for your

images, with the capability to adjust contrast by independently controlling shadows

and highlights within your image, as well as to adjust the overall brightness in ACr

you can make similar adjustments by setting the Exposure, Blacks, and Brightness

sliders however, there are still times you may choose to use levels within photoshop,

particularly if you want to make changes to only one area of your image We’ll talk

about making localized adjustments later in the chapter, but first we’ll cover how to

use levels to make a global adjustment We recommend using levels to adjust contrast

and brightness rather than the adjustment called Brightness and Contrast because the

levels adjustment offers far more accurate control of your settings in addition, you

may opt to use Curves to increase the contrast in a particular range of tonalities rather

than throughout the entire tonal range We’ll talk about Curves later in this chapter

the primary component of the levels dialog box is a histogram display (shown earlier in Figure 6.3) that charts the distribution of tonal values within your image

those values are represented from black at the extreme left to white at the extreme

right this gradation of tonal values appears as a gradient bar along the bottom of the

histogram chart the shape of the histogram chart tells you about the distribution of

tonal values within the image For example, histogram data that is shifted toward the

left indicates that the image is generally dark however, that doesn’t necessarily tell

you anything about the quality of the image; it may simply be a dark scene similarly,

a brighter image has a histogram shifted toward the right

the key things to watch out for on the levels histogram are clipping and

gap-ping Clipping is an indication that information has been lost in the highlights or

shad-ows of your image Gapping is represented by gaps in the histogram and indicates tonal

values that are not represented in your image

Clipping is indicated on the histogram display by data running off the end of the chart Clipping may be displayed in two ways one is as a thin spike at one end

of the chart this is most commonly seen at the highlight end and is often caused by

specular highlights within your image, such as reflections from water, glass, or metal

in other words, it isn’t necessarily a major problem within the image because you

don’t expect to see detail in such highlights

that some pixels in your image are pure black or pure white, but clipping in an RGB histogram can occur when one or more channels has a value of 0 or 255 In such cases, there may or may not be black

or white pixels in your image For example, pure red would have RGB values of 255, 0, 0 and would indicate potential clipping at both ends of the histogram, even though the pixels are pure red, not black or white The histogram in Levels shows RGB information or individual histograms for each of the channels

the other type of clipping is more likely to represent a problem within your image, especially if it occurs in the highlights where it is usually important to retain detail in this type of clipping, the data of the histogram gets cut off abruptly at the end of the chart (see Figure 6.4) rather than ending gracefully before the chart ends

if you think of the histogram as representing a mountain range, ideally the mountains should gradually drop down to the flatland before the chart ends if instead the moun-tains end suddenly in a steep cliff, detail is lost in the area that would have gradually lowered to the base of the chart All pixels within the “missing” tonal values have been clipped to the minimum (pure black without detail) or maximum (pure white without detail) value at that end of the histogram

Figure 6.4 Clipping of data in your image is

exhibited by a histogram chart that appears

“cut off” at one end, as with the shadows here

tonalities in your image Not all subjects will have “mountains” as the correct shape of their histogram;

some may have an even distribution of pixels throughout the tonal range, others may have several

“mountains,” and so on Refer to Chapter 1 for more information about histograms

ideally, your image shouldn’t exhibit any clipping when you get started with your adjustments if it does, it is often preferable that the clipping occur in the shad-ows rather than in the highlights, because your eyes are usually more forgiving of lost shadow detail in a photographic image than of blown highlights however, be careful

not to produce excessive clipping due to inadvertently creating additional contrast as a

result of your adjustments in levels

example, if you are shooting or optimizing a scenic and the sky is a very light and boring gray, you may choose to let the sky become completely white to make it easier to replace If you do, make certain you’re not also blowing out highlights in the rest of your subject matter Similarly, if you are optimizing

an image with a silhouette, you may deliberately make the silhouette clipped black

Another potential problem to be aware of as you make adjustments is gapping

in the histogram think of the histogram chart as a bar chart consisting of many

nar-row bars so that the final result typically looks like a curving data display rather than

one composed of individual bars however, when gapping occurs, you start to see the

individual bars that create the data display, as shown in Figure 6.5 gapping indicates

that certain tonal values are not represented by any pixels in the image (or are

repre-sented, but only by very few pixels)

Figure 6.5 Gaps in the histogram indicate tonal

values that are not represented within the image

gaps in the tonal values indicate that smooth and subtle transitions between tones and colors within the image may be compromised instead of making a gradual

change from one value to another with 10 values in between, for example, the

transi-tion may be from one value to another without any transitransi-tion values between them

this lack of smooth gradations is referred to as posterization (see Figure 6.6) this sort

of flaw isn’t often seen out of the camera, but rather by making strong adjustments in

photoshop, particularly in 8-bit images it can also develop after resaving jpEgs

than the 256 represented by the histogram display Sixteen-bit files have 65,536 tonal values per color channel available, compared to 256 values per channel for an 8-bit file As a result, 16-bit files can lose

a significant number of tonal values without obvious gapping or posterization

however, gaps in a histogram are not an absolute indication of a serious prob-lem with your image minor gaps only a few pixels wide, representing just a few tonal

values, aren’t likely to be visible to the human eye in fact, it isn’t until the gaps become

relatively extreme (at least 10 tonal values) that they are likely to be potentially visible in

the final output Although gaps certainly indicate a potential problem, they don’t define

image quality by themselves if you have significant gapping in an image, use caution

not to make extreme adjustments that may worsen the situation, and closely evaluate the

final image at 100% magnification to ensure there isn’t visible posterization

Figure 6.6 Posterization is represented by a lack of smooth gradations within an image.

So if a histogram exhibits gapping, consider it a caution to carefully check the appearance of your image, but don’t consider gapping a problem if the image looks good

Revealing Detail

nature photographers are often focused on the detail within the image, so tonal adjust-ments frequently revolve around revealing and enhancing detail and texture in the photo the levels and Curves adjustments allows you to do exactly that by enhancing contrast and adjusting brightness to reveal the desired level of detail while maintain-ing an appropriate tonality within the image the chances are you’ll have made these global adjustments in ACr, but we’ll describe making global levels adjustments first and then explain how to limit them to certain parts of the image

Begin by creating a new adjustment layer for levels by clicking on the icon

in the Adjustments panel to access the levels dialog this adds a new levels adjust-ment layer to the layers panel, and the levels dialog box appears (Eleadjust-ments users should click the Create Adjustment layer icon at the top of the layers panel and choose levels.)

background image This decreases the potential image degradation that occurs as changes are made to the image, as well as making it easy to modify the adjustments To avoid any confusion when creating

an adjustment, we recommend accessing the adjustment layers from the Adjustments or Layers panel rather than from the main menu

For most adjustments with levels, you need to adjust only three controls; you’ll find all three directly below the histogram display in the levels dialog box:

t

• he black-point slider (for shadows) is at the far left

t

• he white-point slider (for highlights) is at the far right

t

together, these controls allow you to adjust the overall contrast (by shifting the black-point and white-point sliders) and brightness (with the midtones slider) of your

image with excellent control

We recommend establishing overall contrast before fine-tuning brightness there-fore, start with the black-point and white-point sliders these provide contrast

adjust-ment by allowing you to vary the amount of adjustadjust-ment being applied to the shadow and

highlight areas of your image As a result, you can, for example, sacrifice more detail in

the shadows to improve overall contrast without losing significant highlight detail

As a general rule, most nature images benefit from having the brightest pixel value set to white and the darkest pixel value set to black, to maximize contrast and tonal

range within the image and to ensure that as much detail as possible is visible in other

words, after making adjustments your histogram should stretch (in most cases) nearly

the full width of the chart, with few gaps in that range obviously there are exceptions to

this, but it is a good basic rule Because you know that the last data point at either end of

the histogram chart represents the darkest and brightest pixels, you could make a basic

adjustment by dragging the black-point and white-point sliders inward to the point where

the data begins at each end of the histogram (see the example in Figure 6.7)

Figure 6.7 A basic start with Levels involves

bringing the black-point and white-point sliders in

to where the data begins on the histogram

of course, this is a somewhat arbitrary way to approach an image Although it indeed produces good results for most images, it isn’t an ideal solution for everyone

We recommend using clipping previews to help you set the black and white points

The Clipping Preview

Although a basic visual evaluation of your image while making adjustments with lev-els is certainly effective, it can be even more helpful to use the clipping preview display available in levels this display allows you to see exactly where you are losing detail within your image as you adjust the black-point and white-point sliders As a result, you can make a much more informed decision about the settings you’d like to use for these sliders

When you start with an image that lacks strong contrast and you want to maxi-mize the contrast without sacrificing detail in highlights or shadows, the clipping pre-view display allows you to see exactly where you’ll begin to lose detail based on your specific adjustment of the black-point and white-point sliders

We recommend adjusting the white point first, simply because highlight detail tends to be the more critical adjustment to enable the clipping preview display, hold the Alt/option key while you adjust the highlight slider/white point Your image display initially changes to a completely (or almost completely) black display this indicates that

no pixel values (or very few) are clipped to white before you make any adjustment As you continue to hold the Alt/option key, slide the white-point slider to the left You’ll see more pixels showing up as you move the slider, as shown in Figure 6.8 As a general rule, we recommend adjusting the white point until pixels just start showing up in the clipping preview, and then back off just a hair this is the point where you’ve maxi-mized contrast and tonal range within the image, while sacrificing minimal highlight detail of course, the benefit of the clipping preview display is that you’re able to make

an informed decision about the amount of detail you’re sacrificing to achieve the level

of contrast you’d like to see and about that detail’s location

Figure 6.8 Holding the Alt/Option key gives you a clipping preview as you adjust the white-point slider in Levels, so you can see

where you start losing highlight detail and in which areas of the image

chan-nels that are losing detail within the image The pixels won’t appear as pure white or black in the image until the clipping preview shows those colors However, if any channels are clipping, you may be losing some detail in those areas

the process for setting the black point is nearly identical: hold the Alt/option key while adjusting the black-point slider, and a similar clipping preview appears,

except that now it starts completely (or almost completely) white, with pixels showing

up to indicate where you’re losing shadow detail (see Figure 6.9) As discussed

previ-ously, we are generally willing to sacrifice more shadow detail as opposed to highlight

detail to maximize contrast the clipping preview allows you to make an informed

decision about how much detail you’re giving up with a particular adjustment and the

location of that detail, so you can better determine the extent to which you can push

the black point to produce the desired contrast level

update to reflect those changes

Figure 6.9 The clipping preview, while adjusting the black-point slider, shows you where you’re losing shadow detail in the image.

After you’ve adjusted the black and white points by using the clipping preview, you’re ready to adjust the midtones slider for overall brightness Because this doesn’t

affect the extreme tonal values within the image, there isn’t a clipping preview for the

midtones slider You need to rely on a visual evaluation of the image for this

adjust-ment think of this slider as a brightness control moving the slider changes which

pixel value within the image should be mapped to a middle-gray tonal value, but the

result is a brightness shift this adjustment doesn’t have any rule of thumb you can

follow in terms of positioning the slider at a particular point along the histogram

chart, so you need to make a decision based on a visual review of the image

Within the levels dialog in photoshop (but not Elements) there is a drop-down menu offering access to a number of presets We normally advise adjusting the settings yourself, although the presets can offer a starting place to experiment with different effects What can be more useful, though, is the ability to create your own presets

if you’re working with a series of images that need a certain type of adjustment, click

on the fly-out menu and choose the save levels preset name the preset in the next dialog and click save Your custom preset will then be available from the drop-down menu

avail-able from the fly-out menu in the Levels dialog On some occasions this can be helpful since it can apply

a Levels adjustment differentially to each channel with a single click

companion website, www.sybex.com/go/photoshopnature Use the clipping preview to adjust the white point and the black point, and adjust the midtones slider visually

photography is very much a visual pursuit, so it makes sense to perform a visual review of the image and decide whether you’re happy with the results of the adjust-ment you’ve made You may want to back off the adjustadjust-ments slightly in some situa-tions to minimize the risk of introducing excessive contrast, or in other situasitua-tions you may want to bring the sliders in just a bit farther to produce stronger contrast it’s up

to you to determine the best adjustment for a particular image

settings in the dialog box return to their default values

When you’ve adjusted all three sliders, you’ve finished the basic tonal adjust-ment with levels As with any other adjustadjust-ment layer, if you change your mind about the adjustment at a later time, you can simply double-click the thumbnail icon for the levels adjustment layer on the layers panel, and the dialog box appears, with the slid-ers positioned exactly as you left them

to automatically set the black, white, and neutral values We’ll talk more about those in Chapter 7 when

we talk about color casts

Targeting Adjustments Using a Layer Mask

one of the reasons for using the tonal adjustments within photoshop in addition to

those you already made in ACr is so that you can apply the changes to only certain

areas of the image You may want to do this to help emphasize your subject or to reveal

detail in areas of your picture that have different lighting doing so requires the use of

a layer mask to target the adjustment to a particular area of the image

Every adjustment layer comes with a layer mask it’s the white box that appears

in the adjustment layer in the layers panel, next to the icon for the type of adjustment

the shape of the mask matches the shape of your image, as shown in Figure 6.10 When

a mask is active, there is a thin box surrounding the mask icon the layer mask in

adjustment layers is active by default

Figure 6.10 Every adjustment layer

automatically comes with a layer mask

think of a layer mask as a visibility control for the changes you make on that

layer By default the mask is white Wherever a layer mask is white corresponds to

areas of your image where the changes made on that layer will be visible Wherever a

layer mask is black corresponds to areas of your image where the changes will not be

visible that’s the key to understanding layer masks imagine superimposing the layer

mask over your image—the areas underneath white parts of the mask will show the

changes you make on that layer, and the areas of the image underneath black parts of

the mask will show no effects of that layer

there are three ways to make parts of the layer mask black that are particularly useful for nature photographers the first is by creating a selection before you make

the adjustment layer, the second is by using the Color range option within the masks

panel, and the third is by painting parts of the layer mask black We’ll talk about all of

these approaches in more detail (since Elements users don’t have a masks panel, they

have only two ways to define a layer mask.)

talking about layer masks on adjustment layers, so the mask determines where the adjustment applies and where it doesn’t White on the layer mask indicates areas where the adjustment layer affects the image, and black indicates areas where it does not