Adjusting Exposure and Contrast | Real World Aperture

Aperture provides several types of effects for editing color, tone, and contrast. As with all of Aperture's adjustments, these effects are nondestructive, so you can alter or remove them at any time.

In this section, we'll explore each control individually. Later in this chapter, we'll explore how they work together.

Using the Histogram

Many tone and contrast adjustments are easier to perform with the aid of a histogram. Aperture's Adjustments panel provides a full-featured histogram, which you can activate by choosing Show Histogram (it looks like a little gear) from the action menu in the upper-right corner of the Adjustments panel.

A histogram is nothing more than a bar graph of the tonal values in your image, with black represented by the left side of the graph, white represented by the right side of the graph, and all other tones falling between (Figure 5.13).

Figure 5.13. A histogram provides a bar graph of the distribution of tones in your image. Black is represented by the left side and white by the right.

Histograms provide an easy way to determine whether your image is over- or under-exposed. If the data in the histogram collides with the right side of the image, leaving a spike on the brightest tone, then your image has been overexposed, meaning that it contains a lot of data that is pure white (Figure 5.14).

Figure 5.14. This overexposed image shows a large spike on the right side of the histogram, indicating that the highlight tones have been clipped and will appear in the image as white pixels.

Detail in an image is a function of contrast. You see detail only where there are pixels of different colors and tones. If an area has blown out to complete white, then that area will have no detail at all.

Similarly, if an image is underexposed, you will see areas that have dropped down to complete black (Figure 5.15). An image can usually suffer solid black areas better than solid white areas, because an undetailed shadowy black area will simply look like a very dark shadow.

Figure 5.15. An underexposed image has clipped pixels on the left side of the histogram.

Note

When the tones in your image are cut off by either the right or left side of the histogram, your image data is said to be clipped. So an overexposed image has clipped highlights, and an underexposed image has clipped shadows.

You may think, "Why do I need a histogram to tell me something I can spot by just looking at the image?" However, you'll probably find identifying an over- or underexposure in a histogram much easier and faster than searching images to spot areas of solid white or black. For example, if you've shot a bracketed series of images, you can quickly thumb through them and examine their histograms to see which were over- or underexposed. Also, your monitor does not necessarily display an accurate representation of the tones in your image. If your display is too bright or too dark or if its colors are skewed, you may not be able to accurately judge over- and underexposure simply by looking at a image.

Performing Image Analysis

For the most part, an image editing program does nothing more than allow you to perform very complex brightening and darkening of your image. Your image editor's power derives from the fact that these changes in brightness can vary from pixel to pixel; you can brighten darker tones to bring out more detail in a shadowy area without simultaneously brightening lighter tones, which might result in a loss of detail in your highlight areas.

However, your image editor can never create data. For example, when you brighten shadow tones in an image, the values representing those tones are increased. After the edit, you'll see the new tonal values sitting on a different part of the histogram (for instance, if you've brightened shadow tones, the tones will have shifted to the right; Figure 5.16).

Figure 5.16. Here, the image shown in Figure 5.15 has been brightened. Notice that most of the tones in the image have shifted to the right.

Note, though, that after these tones have shifted, the area where the adjusted tones used to be will contain far less data. Because your image editor cannot make up new image data for data that has ended up in another part of the histogram, you sometimes have to choose your edits very carefully lest you move too much data out of one part of your image.

Therefore, to ensure that you have a lot of editing latitude, you ideally want to work on images that are rich in data, so that you have more tonal information available to push and pull around.

Images with greater contrast usually contain more data than images with low contrast. (We say "usually" because a black-and-white checkerboard is a very high-contrast image, but it doesn't contain a lot of intermediate tones between black and white, so it doesn't take much data to represent.)

Contrast is a measure of the space between the brightest and darkest tones in an image. In a histogram, you can easily assess contrast by noting how much space there is between the brightest and darkest data points (Figure 5.17).

Figure 5.17. As you can see from the histogram (and the image itself), the image on the left has less contrast than the one on the rightthe range of data in the image does not extend over as wide an area of the histogram. The right image has much more latitude for editing.

Note that the overall shape of the histogram is irrelevant. There's no good or bad histogram shape, and there is no objective "good" distribution of tones. The histogram simply shows you where the tones in your image are and how many you've captured.

As you'll see in the next two chapters, you'll often use the histogram to develop a plan of attack for your image editing. Many of Aperture's adjustments provide overlapping functionality. With the histogram, you'll be able to better assess which adjustments you should use at a particular time, and in what order.

Adjusting Exposure

The Adjustments panel always includes an Exposure brick that contains five basic controls for adjusting the exposure in your image. These are the workhorse controls that you'll use to start most of your adjustments. You'll have an easier time understanding what these controls do if you activate the Adjustment panel's histogram display.

With the controls in the Exposure brick, you'll adjust the overall contrast, brightness, and tone in your image. Colors have brightness and tonal values also, so you'll also use these tools to adjust the color in your image.

By default, the Adjustments controls are not selected. Adjusting any slider will automatically check the Adjustments check box.

Using the Exposure Slider

The Exposure slider changes the overall brightness of an image, allowing you to brighten or darken the selected picture. As you move the slider to the right, the data in your histogram slides to the right. As you move the slider to the left, the data shifts to the left (Figure 5.18). The Exposure slider does not provide any kind of constraints to keep you from clipping your data; if you move the slider far enough, you can clip your highlights or shadows.

Figure 5.18. As you adjust the Exposure slider, the data in your histogram will shift left and right. Here, the image at the left has normal exposure; the one in the middle is underexposed, and the one at the right is overexposed.

The numbers in the Exposure slider readout represent stops; if you move the Exposure slider to 0.5, then you've increased the exposure of your image by half a stop.

The Exposure slider provides some additional functionality if you're working with raw files, which we'll explore in the next chapter.

The Exposure Slider for Photoshop Users

If you're accustomed to using Photoshop, you'll find that the Exposure slider in Aperture works just like the Exposure slider in Photoshop Camera Raw.

The Aperture Exposure slider is also analogous to the white point slider in the Photoshop Levels dialog box. When you shift the slider to the right, you're effectively doing the same type of adjustment as when you move the Levels white point slider to the left.

How Aperture's Sliders Work

As you've just seen, Aperture's sliders work pretty much the way you might expect them to work. Slide them to the right, and their associated value increases; slide them to the left, and the value decreases. In Exploring the Exposure slider, you probably discovered that it has a range of -2.0 to +2.0. However, the Aperture Exposure control actually has a range of -9.99 stops to +9.99 stops.

All of Aperture's sliders include a lot of range beyond what the basic slider control provides. To access this extra range, hover the moue over the value field for the parameter you want to adjust. The cursor will turn into an I-beam cursor (Figure 5.19). With this cursor, you can now drag in the value field for the parameter you want to adjust. As you drag, the values will increase or decrease (depending on whether you drag right or left) beyond the values that you can normally access with the parameter's slider.

Figure 5.19. You can access additional range for Aperture's sliders by hovering the mouse over the readout field for a slider and then dragging left or right.

The idea behind this mechanism is that the slider provides control for the parameter's "sweet spot" the range that you will use most often. The extended range provides access to the full range of the parameter. The practical upshot is that, because it covers less range, the normal slider is far more sensitive to subtle changes, making it easier to fine-tune your adjustments.

When dragging in the value field, you can use two modifier keys to control how much the value changes as you drag:

To change the value in large increments, hold down the Option key while you drag in the value field.
To change the value in small increments, hold down the Shift key while you drag in the value field.

You can also adjust the value of any parameter in two other ways:

You can type values for any parameter by double-clicking the field and entering a new value.

Using the Saturation Slider

You can increase or decrease the saturation in your image by moving the Saturation slider in the Exposure brick to the right or left. The Saturation slider has a value of 0 to 4. At 0, all color will be removed from your image. With a value of 1, your image will show its original color. At 4, the saturation in your image will be increased by 400 percent (Figure 5.20).

Figure 5.20. Using the Saturation slider, you can decrease or increase the saturation of the color in your images. Here, the image at the left has normal saturation; the one in the middle is undersaturated, and the one at the right is oversaturated.

Using the Brightness Slider

Like the Exposure slider, the Brightness slider brightens or darkens your image. When you slide the Brightness slider to the right, your image brightens, and the data in the histogram shifts to the right. However, unlike with the Exposure slider, Aperture will try to leave the right edge of the histogram where it is and proportionally adjust all of the other tones (Figure 5.21).

Figure 5.21. The Brightness slider shifts the tones in your image, but tries to keep the white point locked down. Other tones are redistributed proportionally. Compare the original image (left) to the darkened (middle) and brightened (right) images.

Similarly, if you move the Brightness slider to the left, Aperture will darken the tones in your image while trying to preserve the darkest, shadow tone. To put it another way: the Brightness slider adjusts only the midtones in your image, leaving the highlights and shadows alone. The overall shape of your data remains mostly undisturbed, so that the tonal relationships in your image are preserved.

Though the Brightness slider tries to leave the white point where it is, note that extreme brightness adjustments can shift your white point into overexposure, so you'll want to keep an eye on your histogram as you work.

The Brightness Slider for Photoshop Users

The Aperture Brightness slider works just like the Brightness slider in Photoshop Camera Raw. It also performs the same function as the gamma, or midpoint, control in the Photoshop Levels controls.

Using the Contrast Slider

The Contrast slider increases or decreases the contrast in your images by stretching or compressing the data in your image. It increases the contrast in your image by brightening the highlights in your image while simultaneously darkening the shadows. All of the intermediate tones are proportionally adjusted to maintain the overall tonal relationships in your image (Figure 5.22).

Figure 5.22. You can increase the contrast in an image by sliding the Contrast slider to the right (left). Sliding to the left decreases contrast (right).

As with the Brightness and Exposure sliders, when using the Contrast slider, keep an eye on the histogram to make sure that you don't clip your highlights or shadows.

Tip

You can reset any adjustment to its default values by clicking the Reset to Default icon to the right of the adjustment name.

Highlighting Hot Areas

Aperture provides another tool for identifying overexposed areas of your image. When you choose View > Highlight Hot Areas or press Option-Shift-H, you toggle Aperture's Hot Area display.

Aperture highlights all clipped highlights in red, making it easy to identify which areas have blown out (Figure 5.23). As you make exposure adjustments, the red should disappear, assuming that you're making adjustments to try to reduce clipped highlights. If you're trying to achieve an overexposed look, then the red will increase.

Figure 5.23. You can easily identify exactly which areas in your image are overexposed by activating Aperture's Highlight Hot Areas feature. Overexposed areas are shown in red.

Adjusting Levels

By default, Aperture also includes a Levels adjustment in the Adjustments panel. Levels allows you to make adjustments similar to those you can make with the Exposure controls, but adds some power.

You'll use the Levels adjustment when you want a finer degree of control over your tone and contrast adjustments, and when you need to change the black point in your image (the Exposure adjustment provides no control over black point).

Here's how to adjust levels:

Click the reveal arrow in the Adjustments panel to access the Levels parameters and controls. If the Adjustments panel does not contain a Levels adjustment, you can add one from the + menu.

The Levels adjustment provides a histogram display of its own, along with several additional controls (Figure 5.24).

Figure 5.24. The Levels control gives you even more adjustment power than the Exposure control.

Check the Levels check box to activate the control.

Aperture displays a histogram of your image. This histogram display shows a separate histogram for each color channel in your image, as we'll discuss later.

You're now ready to configure the Levels parameters.

Beneath the Levels histogram are three sliders: for the black point, midpoint, and white point. Earlier, you learned that black values are displayed on the left edge of the histogram, and white values are displayed on the right edge of the histogram. The same holds true for the histogram in the Levels adjustment. However, using the sliders below the Levels histogram, you can reassign the values in your image, specifying which should be black and which should be white. Aperture will then remap the in-between values while preserving their overall tonal relationships.

Adjusting the Black Point

In Figure 5.25, the Black Point slider has been moved to the right. Now, tones that were previously dark gray are black. All other tones have been stretched and moved to maintain the same overall relationship. The dark tones in the image will get darker, but because the white point didn't move at all, the highlight tones remain the same.

Figure 5.25. Using the Levels adjustment, the black point of this image has been shifted to the right.

Unlike in the Exposure histogram, the data in the Levels histogram doesn't move. Instead, the original histogram is always displayed. Notice that the original position of the black point is indicated above the histogram in the Levels adjustment; the new position is indicated below. The main histogram display at the top of the Adjustments panel shows the final, adjusted histogram.

In this particular example, we've chosen to clip the black point a little bit, to achieve increased contrast and saturation.

Adjusting the White Point

You can change the white point in your image by sliding the White Point slider to the left. In Figure 5.26, the white point was shifted to brighten the image.

Figure 5.26. This image was brightened by moving the white point inward, thus redefining pixels that were previously light gray as white.

You can improve the overall contrast of an image by dragging the white point and the black point both inward.

Adjusting the Midpoint

Notice that whenever you move either the white point or black point, the midpoint makes a corresponding move to preserve the overall tonal relationship in your image. By moving the midpoint, you can control the brightness of the middle tones in your image (Figure 5.27).

Figure 5.27. Shifting the Midpoint slider back and forth brightens or darkens the midtones in the image.

At the top of the midpoint is an additional slider. You can move this slider left to right to specify the tone that you want to edit. Thus, the Midpoint slider does not necessarily adjust only the midpoint. You can easily move it to adjust a different part of the histogram (Figure 5.28).

Figure 5.28. Moving the upper point of the Midpoint slider changes the part of the tonal range affected by the slider.

The Midpoint slider is a deceptively simple tool that provides a tremendous amount of editing power. The easiest way to use the Midpoint slider is to first move the bottom slider to either brighten or darken the midpoint. Then adjust the top slider to focus on the exact part of the tonal range that you want to adjust. Then fine-tune using the bottom slider again.

Using Auto Levels

A very common Levels adjustment is moving the white point and black point so that they sit right at the edge of the data in the histogram. This adjustment improves contrast overall and makes a good starting point for further edits. Aperture's Levels control includes an Auto Levels button (Figure 5.29) that makes this adjustment for you.

Figure 5.29. The Auto Levels button automatically sets the white and black points in the Levels dialog box.

There's no complex analysis here; Auto Levels simply looks for the brightest piece of data in the histogram and sets the white point there and then sets the black point to the darkest piece of data (Figure 5.30).

Figure 5.30. Clicking Auto Levels adjusts the white and black points, giving the image (right) stronger contrast.

If you choose Show Auto Adjust Options from the Levels adjustment action menu, two additional sliders appear beneath the Auto Levels button (Figure 5.31). These let you control the amount of the white and black data that the Auto Levels adjustment clips. These sliders have no effect on an adjustment that has already been made.

Figure 5.31. The White Clip and Black Clip sliders let you specify the amount of data that the Auto Adjust button clips.

Adjusting Quarter Tones

Aperture's Levels adjustment lets you add two more sliders: one between the black point and the midpoint, and one between the midpoint and the white point. You can activate these sliders by clicking the Quarter-Tone Controls button in the Levels brick.

These additional control points work just like the midpoint controls. You can drag the bottom slider to brighten or darken a particular range of tones in your image, and you can adjust the top slider to specify which tones. In Figure 5.32, we want to darken some of the shadow tones, but if we use the Black Point slider, the shadows will become too dark; instead, we can drag the Quarter-Tone slider that sits between black and the midpoint to the right to darken the quarter tones. This move tells Aperture to brighten the tones beginning at 75 percent black.

Figure 5.32. Moving the Quarter-Tone slider to the right brightens the darkest quarter tones in the image.

This adjustment works okay, but it still darkens a lot of the shadows under the plants. It would be nice to brighten the tones that are a little bit brighter than 75 percent black. If we slide the upper control of the shadow midpoint to the right, we can do just that, targeting some slightly lighter tones (Figure 5.33).

Figure 5.33. Shifting the upper point on the shadow quarter tone changes the tone that is being adjusted.

Working with Levels Instead of Curves

If you're a Photoshop user who's used to using the Curves dialog box for your tonal adjustments, you'll probably find that Aperture's Levels adjustment is an excellent substitute. Because it provides five editable points, three of which have changeable input values (the midpoint and quarter-tone controls), you should be able to perform almost all of the most common Curves adjustments that you can make in Photoshop.

So before you take an image into Photoshop to apply a Curves adjustment, spend some time getting familiar with Aperture's Levels adjustment. You may find that it's all you need.

Adjusting Highlights and Shadows

So far, the adjustment tools that you've used in Aperture simply brighten or darken a particular range of tones. While you have some control over which range of tones are affected, all tones that fall within that range are adjusted, regardless of where they are in the imagethat is, when you change the black point with a Levels adjustment, not only do you alter the dark tones of the shadows in your image, you also alter any other pixels with those values anywhere else in your image. However, your image may contain pixels that aren't actually shadows but that have those same tonal values, and they too will be adjusted (Figure 5.34).

Figure 5.34. Making a quarter-tone adjustment using Levels brightens the street-level shadows in this image. Unfortunately, it also brightens all of the other pixels in the scene that have those same values, resulting in a loss of contrast in the building's details.

The Highlights and Shadows adjustment applies a little more intelligence to its brightening and darkening efforts. Rather than blithely working through your image, adjusting the brightness of any pixel that falls within a particular tonal range, the Highlights and Shadows adjustment first analyzes the surrounding pixels for each pixel in your image. If it finds that the surrounding pixels have similar values, then it adjusts the candidate pixel as you've specified. But if it finds that surrounding pixels are lighter or darker, then it rolls off the effect that you've specified. By default, the Highlights and Shadows adjustment analyzes a radius of 200 pixels around each pixel in your image.

So if you're trying to brighten shadows with the Highlights and Shadows adjustment, Aperture will figure out where the shadow turns into a midtone and won't apply as much of an adjustment there, creating a much more realistic-looking adjustment that blends smoothly into the midtones that aren't being altered (Figure 5.35). In addition, other tones in your image that have shadow values but that aren't surrounded by lots of shadowy pixels will probably not be adjusted.

Figure 5.35. With the Highlights and Shadows adjustment, Aperture identifies which dark tones are shadows and adjusts them, leaving other equivalent dark tones in the image alone.

The same sort of analysis is performed when darkening highlights with the Highlights and Shadows adjustment.

To add a Highlights and Shadows adjustment, choose Highlights and Shadows from the + menu in the upper-right corner of the Adjustments panel. The adjustment is added to the current primary select. You can also add the adjustment by pressing Control-H.

Note

If Aperture beeps when you press Control-H, it's probably because your image has not finished loading. Wait for the image to load and then try again.

Adjusting Shadows with Highlights and Shadows

To brighten the shadows in your image, drag the Shadows slider in the Highlights and Shadows brick to the right. The farther you drag, the more you will brighten your shadows (Figure 5.35).

You have little control over what is considered to be a shadow area. Aperture will decide this on its own as it analyzes your image. The Shadows slider has a range of 0 to 100, with 0 being no adjustment at all. You cannot darken shadows with the Shadows slider; you can only brighten them.

Performing an extreme Shadows adjustment can give you a better idea of how the Shadows slider works. For example, Figure 5.36 shows a full-strength adjustment of 100. The shadow tones in the background of the image have been brightened (to the point of washing out), but there's a broad halo of darker pixels around the bird. As Aperture analyzed the image and found the bright pixels of the bird, it applied less brightening to the shadow pixels around the bird, to try to create a smoother transition from the background to the foreground.

Figure 5.36. This overdone Shadows adjustment lets you see how the Highlights and Shadows command applies varying amounts of adjustment.

When making the Highlights and Shadows adjustments, keep an eye out for these types of halos. Your goal is to brighten the shadows so they are closer to the midtones, but without ever making them brighter than the midtones.

Adjusting Highlights with Highlights and Shadows

To darken highlights in your image, drag the Highlights slider to the right. As you drag, the highlights will darken (Figure 5.37).

Figure 5.37. You can darken the highlights in an imagein this case, the details on the bright feathersby dragging the Highlights slider to the right.

As with the Shadows slider, it's possible to create strange halos with the Highlights slider. Again, your goal is to darken the shadow tones to make them closer to the midtones, but without making them darker than those midtone values.

Using Advanced Controls

If you open the Advanced option in the Highlights and Shadows brick, you'll find some additional controls.

Radius: As mentioned earlier, the Highlights and Shadows adjustment by default analyzes a radius of 200 pixels around each pixel that it is adjusting. Using the Radius slider, you can increase or decrease this analysis radius. When you use the Highlights and Shadows adjustment on a shadow that is adjacent to a bright area, the edges of the shadow will often brighten more slowly than the middle, resulting in shadows with dark edges (Figure 5.38). By reducing the radius, you tell Aperture to analyze a smaller area for each pixel, yielding a more even adjustment. The result is a shadow with more even tone.

Figure 5.38. By reducing the radius, you can create a shadow with a more even tone.

Color Correction: Though you may think of shadows as simply "dark," shadows actually have a color that depends on the color of the light that's casting the shadow. The sun, for example, casts shadows that have a blue hue. The darker the shadow, the deeper the hue.

As you brighten a shadow, its hue needs to get warmer to appear realistic. The Color Correction slider lets you add more saturation to the tones that are altered by the Highlights and Shadows adjustment. In this way, you can create shadows that have a more realistic tint.

The Color Correction slider also works on pixels that have been brightened with the Highlights slider, but the results are is far more noticeable on shadow adjustments.

High Tonal Width and Low Tonal Width: You can widen or narrow the range of tones that Aperture considers to be highlights or shadows by adjusting these two controls. For example, if you're darkening highlights and find that Aperture is not darkening all of the highlights that you want, then try increasing the High Tonal Width value. Similarly, if you find that Aperture is adjusting too many highlights, then try decreasing the High Tonal Width value. Low Tonal Width performs the same adjustments for shadows (Figure 5.39).

Figure 5.39. By increasing the Low Tonal Width value, you broaden the range of tones that the Highlights and Shadows adjustment affects.

Mid Contrast: Although the Highlights and Shadows adjustment is predominantly used to alter the highlights and shadows in images, it can perform some adjustments to the midtones of a picture. The Mid Contrast slider lets you increase or decrease the contrast in your image (you'll see your histogram stretch out accordingly) and allows you to create smoother transitions from your altered highlights and shadows and your midtones.

Using Highlights and Shadows to Apply Fill Flash

The Shadows slider in the Highlights and Shadows adjustment provides a quick, effective fix for images that have bright backgrounds and dark foregrounds such as portraits that should have been shot with a fill flash. To apply a fill flash effect, add a Highlights and Shadows adjustment and then drag the Shadows slider to the right (Figure 5.40).

Figure 5.40. Applying a Shadows adjustment using Highlights and Shadows makes an excellent substitute for a fill flash.

You usually don't need to use any of the advanced controls for this type of adjustment because it's usually very easy for Aperture to correctly analyze a high-contrast, backlit scene.

Note

Any time you brighten a shadowwith any type of adjustmentyou run the risk of making the noise that's buried deep in the shadows more visible, so when adjusting the Shadows slider, make sure that you don't brighten things up so much that your shadow tones become visibly noisy.

Using Highlights and Shadows to Reduce Contrast

Photographers spend a lot of time trying to achieve better contrast in their images, but too much contrast can be garish and distracting. By using Highlights and Shadows to brighten shadow tones and darken highlight tones, you can reduce the contrast to create a more evenly exposed image (Figure 5.41).

Figure 5.41. Using the Highlights and Shadows adjustment, you can easily reduce contrast in images that are too harsh.

While shooting outdoors in bright sunlight can yield wonderful textures, it often produces images with shadows that are too dark. The Highlights and Shadows adjustment can make short work of these types of corrections.