11.3 Correcting Exposure

Whereas the previous section addressed the specific issue of blown highlights—i.e., bright regions of an image that have been overexposed to the point that details are lost—in this section we deal with the more general issue of adjusting exposure for the entire image.  This includes images that are both over-exposed (too bright) and under-exposed (too dark).  We’ll also consider how to manipulate the lighting patterns in the scene so as to achieve more even lighting, as well as exposing the bird differently from the background.

11.3.1 Fixing Underexposure

Underexposure is one of the most difficult problems to correct in postprocess.  If you follow the ETTR/BETTR protocol (section 6.2), you’re much more likely to overexpose than underexpose, and in general, that’s a good thing, because as long as you don’t clip the highlights, your overexposed images can generally be corrected without loss of information.  Underexposure can, however, occur, even for photographers adhering to the ETTR principle.  One common example is when you’ve been shooting a pure white bird in bright sunlight in manual exposure mode and you suddenly encounter a non-white bird in the shade nearby.  Unless you remember to readjust your exposure settings, this new bird is likely to be massively underexposed.  The figure below shows an example of an extremely underexposed image.

Fig. 11.3.1: An extremely underexposed image, and attempts to correct it. 
Left: the original image.  Center: after increasing exposure digitally in ACR. 
Notice the background noise and the overall poor image quality.  Right:
after reducing noise and increasing sharpness in Photoshop.  Notice that
the colors are quite poor.  There’s a limit to what you can do with images
this massively underexposed; shooting in RAW definitely helps.

     In the leftmost pane of the above figure is the original RAW image.  The middle pane shows the image in Adobe Camera Raw (ACR) after increasing the Exposure slider in ACR to its highest setting.  Notice the enormous amount of noise in the background.  The rightmost pane shows the final image in Photoshop proper, after sharpening and noise reduction.  Though it’s impressive that so much information was able to be pulled out of such a massively underexposed photo, the resulting image doesn’t look great.  The overall color has suffered from the underexposure and the measures that were taken to correct the exposure.  Nevertheless, this example does illustrate the advantage of shooting in RAW: if the original image had instead been a JPG, the degree of information recovery illustrated here would have been utterly impossible.
    The example below shows a more tractible case, in which the image is only slightly underexposed.  In this case we’ve again increased the brightness of the image via the Exposure slider in ACR (note that the Fill Light slider is often useful for this purpose as well, though we didn’t use it here).  Doing so created some blown highlight regions, so we’ve also adjusted the Recovery slider to protect those areas.  The Brightness slider (not shown here) can often be just as useful as the Exposure slider, and sometimes even more so.  The exposure shown here is far from perfect, but is at least brighter than before, and can now be fine-tuned in Photoshop proper.

Fig. 11.3.2: Increasing brightness via the Exposure slider in ACR.  Be careful
to check how the defaults are set for the other sliders, since this can work against
you if you don’t change them.

Correcting exposure in ACR is typically the most effective option, since it’s the option that has the most information available to it.  Once the photo case been converted from RAW to the internal representation used by Photoshop proper, certain information will have been discarded and can’t be reclaimed without going back to the original RAW file.  The next example (below) shows the effect of adjusting exposure via the Exposure tool in Photoshop proper.  As you can see, in order to achieve an overall brightness level similar to that for the previous example we’ve had to blow the highlights in the bird’s cheek and throat.  Whereas in ACR we could guard against this by adjusting the Recovery slider at the same time, once the image is in Photoshop proper (i.e., after it’s been converted from RAW), that’s not possible.  By and large, highlights that you’ve blown via adjustments made in Photoshop proper aren’t directly recoverable.

Fig. 11.3.3: Increasing brightness via the Exposure slider in
Photoshop proper (rather than ACR).  This method often results
in blown highlights, so I don’t recommend it.

Note that in the Exposure tool there is a slider called Gamma Correction.  While the Exposure slider in Photoshop proper tends to be unwieldy for images having bright white regions, the Gamma Correction slider can be very useful, both for increasing brightness and for decreasing it.  In the example below we’ve gone back to the original image and increased brightness by adjusting the Gamma slider in the Exposure tool.  Increasing brightness this way often results in a decrease in the overall contrast of the image, but this can be corrected subsequently via the Brightness/Contrast tool or the Levels tool (we’ll get to those tools in just a minute).

Fig. 11.3.4: Increasing brightness via the Gamma slider in
the Exposure tool.  Reducing gamma increases brightness,
but it can also reduce contrast; you can compensate by
increasing contrast later via the Brightness/Contrast tool
or the Levels or Curves tools.

Note that in all of these examples we’ve made sure that the Preview button is checked.  This allows you to see the effect of your changes immediately.  To compare the new settings to the original image, you can simply toggle the Preview button on and off repeatedly.  This is an extremely useful feature, because it provides context and allows you to judge the effect of a tool relative to the unmodified image.
    In the next example we again return to the original image and correct the exposure using the Brightness / Contrast tool.  Here we’ve increased the Brightness slider substantially, and you can see that it has increased the overall image brightness without blowing the highlights.  We’ll see in the next section that the Contrast slider can be useful for restoring contrast in overexposed images, though there are more flexible ways to adjust contrast, as we’ll see.

Fig. 11.3.5: Increasing brightness via the Brightness/
Contrast tool.

The next tool is one of the most useful tools in Photoshop for adjusting brightness and contrast.  It can also be used as an alternative to the Highlights tool to recover blown highlights.  This is the Levels tool (see below), which shows you the image’s current histogram and allows you to set sliders that reshape that histogram.  The sliders in this case are the small triangles (actually pentagons) below the histogram and below the Output Levels track.  In the current example, we’ve moved the leftmost (black) and middle (gray) sliders below the histogram.

Fig. 11.3.6: Simultaneously increasing brightness and contrast
via the Levels tool.  The triangles/hexagons under the histogram
are the sliders.  The black slider increases blacks, the white slider
increases whites, and the gray slider can increase or decrease
overall brightness, depending on which way you slide it.

The middle (gray) slider adjusts the overall brightness; in the example above we’ve moved it toward the left, which increases brightness.  In order to restore some contrast, we’ve moved the leftmost (black) slider a bit to the right, which darkens darker colors toward black.  Another way to increase brightness is to move the rightmost (white) slider for the histogram to the left; this increases lighter hues toward pure white.  If you slide that white slider too far left (i.e., into the region of the histogram having nonzero height), you’ll induce clipping of highlights, so be careful.  This slider is mainly useful for making sure that the brightest point in the image is true white (rather than gray), which helps to foster the impression of high dynamic range.  The sliders beneath the Output Levels track are less often useful, because they have a fairly drastic effect on the image.  We’ll see later that sliding the white slider all the way to the left is useful for fixing eye-shine, by making the selected region (the eyes) pure black.  We’ll see that there are many uses for the Levels tool; this tool is very simple to use, fast, and often highly effective for adjusting brightness or contrast either in the entire image or in selected regions (as per the D-PIE technique: Differential Processing of Image Elements).
    The next tool is one that I find to be too unweildy for most tasks, though it is a popular tool among avid Photoshoppers.  This is the Curves tool, which allows you to superimpose a curve onto the histogram to indicate how you’d like to transform the brightness profile of the image.  In the example below, we’ve increased the overall brightness of the image by applying a convex curve that concentrates the increase in the middle tones, with progressively less change toward the extremes.  This type of nonlinear transformation is useful for increasing overall brightness without blowing the highlights or softening the blacks. 

Fig. 11.3.7: Increasing brightness via the Curves tool.  I find this
tool very unwieldy, and rarely use it, but others swear by it.

In this particular case the Curves tool has done a very decent job of improving this image, and it took only two seconds to accomplish.  Notice the tiny black square on the curve.  The way we drew this curve was by clicking the mouse on the diagonal line (the one that runs from the lower left corner to the upper right corner) and dragging this point upward until the curve assumed the shape you see above.  Because the Preview box is checked, we were able to see the effect of this transformation on the image in real-time.  Simple adjustments like this can be performed in no time flat using the Curves tool.  Note that you can now refine this curve by clicking anywhere on the curve and dragging that point up or down.  By repeating this process you can create a whole set of anchor points that define an arbitrarily complex curve.  In theory, this is an extremely flexible means of adjusting an image’s histogram.  In practice, it becomes very tedious and frustrating, because the effect of moving each point is tempered by the positions of all the other points you’ve already set.  Also, the working space in the Curves window is limited and it’s hard to make tiny changes at the finest levels.  As a result, the transformation tends to be very sensitive to small mouse movements, making it difficult to achieve subtle changes.  For that reason, I personally tend to use the other tools much more often, particularly Levels, Shadows/Highlights, and the Gamma slider in the Exposure tool.

11.3.2 Fixing Overexposure

For photographers committed to the ETTR technique (Exposing To The Right—section 6.2), it’s much more common to have overexposed than underexposed images.  While the highlight alerts on your camera’s LCD can help you to avoid blowing the highlights, for some photos you’ll find that the resulting image appears too bright, or
washed out.  This can be corrected using any of the tools mentioned in the previous section (as we’ll illustrate shortly).  The advantage of reducing brightness for overexposed (but non-clipped) images over increasing brightness for underexposed images it that the brighter images use larger numbers of bits to encode their pixels, so no information has been lost, unlike the case with underexposure, in which fewer bits are used and numerical precision often suffers as a result.  Also, brighter images tend to be less noisy (as long as they were produced via large apertures or slow shutter speeds, rather than high ISO settings) than underexposed images.
    Reducing brightness in Adobe Camera Raw can be accomplished via the Exposure slider.  In the example below, we’ve decreased the Exposure by -0.45, while also using the Recovery slider to reclaim highlights.  We’ve also increased the Blacks setting and then increased the Brightness to improve contrast; the Contrast slider was also adjusted a bit.  Although the resulting image is far from perfect, these adjustments are appropriately conservative for the RAW conversion phase, which applies to the full resolution image.  Finer adjustments to the exposure can be performed when the image has been reduced to its target resolution prior to distribution (i.e., posting on a web page or printing to photographic paper).  Ideal brightness and contrast, like sharpness and saturation, are very medium-dependent.

Fig. 11.3.8: Fixing overexposure in ACR can be done by decreasing the
Exposure setting.  Though this will certainly reduce overall brightness,
you may still need to increase the contrast of the image, since many
overexposed photos appear washed out.  Here we’ve increased contrast
both via the Contrast slider and also by increasing the Blacks and then
compensating by increasing the Brightness.

In the example below we’ve returned to the original RAW image and adjusted the brightness in Photoshop proper using the Gamma slider in the Exposure tool.  Increasing gamma generally increases contrast and saturation while decreasing brightness.  Like the Curves tool in its most basic use (see below), Gamma Correction applies a nonlinear function that leaves pure white and pure black values unmodified while adjusting mid-tone values (the gamma function is described in more detail in section 16.2).

Fig. 11.3.9: Overexposure can also be adjusted in Photoshop proper,
via the Exposure tool.  Here, we’ve ignored the Exposure slider and
instead increased the Gamma, which simultaneously decreases
brightness while increasing contrast.

In the next example we instead use the Levels tool to decrease brightness and increase contrast.  By moving the gray slider to the right, we’ve deformed the histogram so as to darken the mid-tones, while moving the black slider to the right has increased the dark presence and enhanced the overall contrast.  Note that we have, in effect, adjusted the three sliders so that the black slider is close to the leftmost end of the nonempty part of the histogram, the white slider is close to the rightmost end of the nonempty part of the histogram, and the gray slider is closer to the main mass of the histogram.  Though not infallible, this procedure provides a very general set of guidelines for using the Levels tool.  Just keep in mind that moving the black and white sliders into the interior of the histogram mass will cause clipping of blacks and whites, respectively, in the image.  This is why it’s preferable to keep them at or near the ends of the visible histogram mass.

Fig. 11.3.10: The Levels tool is another favorite method for fixing
overexposure.  Here we’ve moved the midtone slider (gray triangle
below the histogram) to the right, to decrease overall brightness,
and then moved the black slider to the right a bit to beef up the
blacks in the image.

For the next example we’ve achieved roughly the same effect using the Brightness / Contrast tool instead of Levels.  In this case we’ve descreased the brightness and increased the contrast directly via their respective sliders.  Though this appears to be a simpler and more direct means of adjusting these image qualities, the Levels tool with its visual representation of the histogram provides more information to guide you when setting the sliders. 

Fig. 11.3.11:  The Brightness / Contrast tool can also be
useful for decreasing brightness while also adjusting
contrast, which is commonly required for overexposed

In the next example we apply the Curves tool.  Whereas in the previous section we dragged the curve above the diagonal to increase brightness, for this overexposed image we instead drag the curve downward so as to decrease brightness.  Though we’ve again used only a single point, we could have introduced any number of points to arbitrarily shape the curve to our liking.  Note that a silhouette of the image’s histogram is faintly depicted in the background of the curve pane, providing some guidance as to which parts of the brightness spectrum need attention.  The black and white triangles below the curve act as sliders just as in the Levels tool, allowing you to set the white/black points and possibly to clip the histogram at either end.  Thus, the curves tool provides all the information present in the Levels tool, with the added capability to deform the histogram (via the curve) in any arbitrary way.

Fig. 11.3.12: The Curves tool is arguably the most flexible tool
in Photoshop for fixing exposure problems, though I rarely use
it, because I find it unweildy.  In this simple example, we’ve adjusted
the exposure by dragging a point on the diagonal downward, creating
a curve.  Overexposure has been substantially reduced, though the
image now exhibits saturation problems.

Finally, we consider the use of the Shadows / Highlights tool for decreasing brightness.  By increasing the Highlights slider we’ve decreased the overall brightness of the image.

Fig. 11.3.13: The Shadows / Highlights tool is typically not the
best one for fixing overexposure, but in some cases it works.  It
is, however, extremely useful for fixing the blown highlights (not
shown here) that often accompany overexposure.

     It should be clear by now that Photoshop (and Adobe Camera Raw) provide numerous, redundant methods for correcting overexposure and underexposure.  Though some methods are more flexible than others, additional flexibility (via more parameters to be adjusted) is sometimes more of a hindrance than a help.  A good strategy, when deciding which tool to use for a given image, is to try the simpler tools first; because of their simpler interface, it should become quickly apparent whether the simpler tools are sufficient to achieve the desired effect.  However, in order to gain familiarity and proficience with all the tools, it’s a good idea to vary your routine and do some exploration before adopting a single favorite tool.  Remember that when all you have is a hammer, everything starts to look like a nail.

11.3.3 Fixing Uneven Lighting

When shooting on sunny days, the patterns of sun and shade can work for you or against you.  Unless the sun is directly behind you, a bird in direct sunlight will generally have at least some part of its body shaded, and this may or may not look good.  As we described in section 7.6, the use of flash to fill in the shadows is a powerful field technique that, unfortunately, is not always as effective as you may like (due, for example, to distance or exposure constraints).  Fortunately, for many images you can fill in the shadows digitally in Photoshop, using the Shadows / Highlights tool introduced earlier.  In the example below, we’ve used the Shadows sliders to lighten the shadows of both the bird and its environment.  Notice how the deep shadows under the bird’s tail and wings have been lightened to reveal significant detail that was previously hidden.  The shadow on the bird’s face has also been lightened, and much of the bird’s underside is now lit up as if fill flash had been used to light the bird from the front. 

Fig. 11.3.14: The Shadows / Highlights tool in Photoshop can be used to re-shape
the lighting patterns in the scene.  The top image conveys the impression of a strong
light coming from above.  The bottom image, after strong manipulation of shadows,
lessens the effect by creating the impression that light is also shining onto the
bird from other angles as well.  Notice the effect on the mood of the image.

Obviously, the use of fill flash and digital lightening via the Shadows tool aren’t mutually exclusive; indeed, the Shadows tool can be highly effective at enhancing the effect of fill flash in situations where your flash unit isn’t powerful enough to provide enough fill at the given distance.  The use of the Highlights tool can also help to reduce the harsh effect of direct sunlight by mitigating the difference between the sunlit and shadowed regions of an image.  Simultaneously dampening the highlights and lightening the shadows can make an overly-contrasty image appear more uniform, though if taken to the extreme it can result in images that appear flat and dull.  Finding the right balance is a matter of intuition, experience, patience, and luck.
    As with all methods that increase brightness in underexposed regions, the ability to pull out detail is typically vastly improved by shooting in RAW and processing your images in 16-bit mode in Photoshop.

11.3.4 Differential Exposure of Bird and Background

All of the methods described in the preceding two sections can be taken one step further by applying them differently to different parts of the image.  That is, rather than applying any of the foregoing tools to the entire image, you can use your favorite selection tool to select one region of the image (such as just the bird, or just the bird’s wing, or just its head, etc.) and then apply an appropriate exposure tool to just that region.  You can then repeat this for all the different parts of the image.  This obviously takes more effort, but potentially offers greater gain, since you can fine-tune the exposure parameters for each part of the image.  Such fine-tuning, if done well, can in some cases result in images appearing to have higher dynamic range than would normally be possible with today’s DLSR’s, because you can pull the maximal amount of detail out of all parts of the image—including both the shadows and the highlights.  Normally, with today’s DSLR’s you have to choose between maximizing detail in the bright regions of the scene (by exposing darker) or maximizing detail in the dark regions of the scene (by exposing brighter).  We’ll see in Chapter 13 that via the merging of multiple photos having different exposure settings we can achieve a High Dynamic Range (HDR) image having more striking color and tonal ranges and more details in extreme areas of the histogram.  When working with only a single photo, the HDR technique is inapplicable, but by applying different exposure adjustments to different parts of the image, you can sometimes achieve an effect similar to HDR.  This is what I call the D-PIE technique, or Differential Processing of Image Elements.
    The figure below serves as a very rudimentary example.  The top pane shows the original image and the bottom shows the image after the bird had been separated from the background (via separate layers) and the twain processed separately using the Shadows / Highlights tool.  As you can see, the background has been slightly darkened while the bird has been significantly brightened.

Fig. 11.3.15: By first selecting the bird and moving it to a separate layer,
you’re then free to manipulate the foreground and background separately.
In this case we’ve lightened the bird while darkening the background, to
make the bird stand out more.  In this particular case it’s probably been
a bit over-done.  Try to be more subtle when you apply this technique.

In this case the effect is a bit exaggerated for the sake of illustration, but it should suffice to demonstrate the potential of this technique to drastically alter the relative appearance of the bird versus its background.  The background was slightly darkened by dampening the highlights via the Highlights tool—a subtle technique I often use to make the background slightly less distracting.  For the bird, the shadows were lifted via the Shadows tool, while the highlights were very slightly supressed using the Highlights sliders. 
    There are several things to note about the above image.  First, the effect achieved here is again very similar to what could be achieved in the field using stronger flash: recall that flash falls off rapidly with distance, so often the bird is substantially more affected by the flash than the background.  In the case of true flash, the effect is sometimes too strong, and can be moderated by applying the above transformation in reverse (slightly darkening the bird and lightening the background).  Second, note that the changes to the exposure have affected the colors of the bird.  You’ll often find that after modifying the exposure of an image you’ll want to use the Saturation tool to slightly de-saturate the subject; this is especially true when you’ve used any of the exposure tools to darken the bird.  Third, keep in mind that all of the techniques described here have the potential to make the image appear artificial.  Because changes to exposure in postprocess can alter the perception of the lighting patterns in the image, they can confuse the viewer by implying lighting patterns that differ between foreground and background.  For example, if you modify the subject so as to appear to be lit from below, while the rest of the scene is clearly lit from above, you run the risk of making the image appear artificial, as if the bird had been pasted into the scene from a different photo.  This might be the effect you want—or it might not. 
    Another example of the above technique is shown below.  In this case, the background was brightened using the Levels tool, to create the perception of a bright sunrise behind the bird.  The bird was processed separately from the background, and indeed individual parts of the bird were processed separately from each other to create the impression of greater dynamic range.  Some of the whites were exaggerated to make the bird seem to glow.  The red pupil was desaturated and the yellow iris was brightened to make the eye look more alive.  The bird was also sharpened using the Unsharp Mask (section 11.4).  All of the exposure changes in this example were achieved using the Levels tool.

Fig. 11.3.16: Differential processing of image elements.  In this image
the foreground and background were adjusted differentially via the
Levels tool.  In addition, parts of the bird’s plumage were individually
selected and brightened or darkened to give the impression of greater
dynamic range.  This is an extremely powerful technique, though it
requires some care and experience to use it most effectively.

    I often find that exposure changes that seem to look good to me during postprocessing don’t look as good when I come back later after taking a break from the computer.  It’s a good idea to create several versions of an image during postprocessing and then come back later with a refreshed perspective (i.e., fresh eyes) and see which version looks best.  This is easy to do using the Save As... menu option; simply save the file using a new file name each time by affixing a version number to the end (e.g., warbler-6.jpg, warbler-7.jpg, etc.).