Curves is probably the most commonly used tool to bring forth detail from the shadows and highlights. This is likely due to the fact that most photographers are familiar with the tool. It is also fairly straightforward and easy to use. One the other hand, not all photographers may be aware of the limitation of Curves as a shadow and highlight enhancement tool
Figure 1 shows Curves used to enhance shadow detail. Figure 2 shows Curves used to enhance highlight detail.
The first limitation of Curves has to do with the tonal levels. In the first article of this series, it was pointed out that a JPEG file has only twenty levels (approximately) in the shadows before any editing begins. Curves is limited to these levels. Thus, no matter what is done with Curves, the photographer is limited in what can be accomplished because she is dealing with a relatively limited amount of information. Curves simply can not produce shadow detail that isn't there in the first place.
The story is much better in the highlights. There, a JPEG file starts off with about sixty-nine levels before any editing is done. Thus, there is more detail with which to work. More detail produces better results.
When working with raw files, the situation significantly improves. Raw files contain sixteen times more levels that JPEG files. As a consequence, there is more detail in both the shadows and the highlights. This considerably increased number of levels allows better results to be achieved with Curves. This is particularly true in the shadows.
When using Curves, the issue of tonal stretching and compression needs to be considered. Curves stretches the tones that are being enhanced. This has its good points and its bad. The tonal stretching increases contrast (a desired effect that makes the details more noticeable). On the other hand, it increases the distance between the tones. If the tonal stretching goes too far, banding results.
At the other end of the curve, the tones are compressed. This causes a certain degree of quantization error and the resultant loss of tonal levels.
Consequently, this tonal stretching and compression causes a loss in image quality. Figure 3 and 4 demonstrate this point. Figure 3 shows a histogram from a JPEG image that has not been edited. The histogram is smooth with no tonal gaps or compression. Figure 4 shows the histogram after Curves was used to bring out the shadow detail. This Figure shows gaps in the darker tones and compression of the lighter ones. Overall, a degradation of the histogram has occurred. This will be reflected in a reduction of image quality.
There are two ways that the image degradation from the tonal stretching and compression by Curves can be reduced. The most important of these is to shoot in raw. While the use of raw will not eliminate the tonal stretching and compression, the large increase in the number of tones significantly reduces the impact of the stretching and compression. Since there are so many additional tones with raw, the gaps between the tones caused by tonal stretching becomes very small. Additionally, the loss of tones due to tonal compression will not be noticed due to the larger number of tones with which raw starts. This is demonstrated in Figure 5. This figure shows a histogram from the exact same image and curve used for Figure 4. The only difference is that the image was converted from a raw file and saved as a TIFF. This allowed the file to keep the larger number of tones inherent in a raw file. Beyond a shadow of a doubt (pun intended), the histogram of the TIFF image is far superior to that of the JPEG image.
The other way to reduce the negative effects of the use of Curves is to use masks or the Blend If controls. Masks and the Blend If controls can be used to protect the areas that are not being enhanced from the impact of the Curves. For example, if Curves is used to enhance the shadows, a mask could be used to protect the midtones and highlights from the Curves. Thus, the tones in these areas would not be degraded. On the other hand, this method still allows the tones in the enhanced areas to be stretched.
Curves has an advantage in the area of clipping. Unless the end points of the curve are moved up or down the vertical axes, Curves will not clip either the shadows or the highlights. This is easily seen in Figure 6. This figure shows the histogram after a rather extreme curve was applied to an image to dramatically lighten the shadows. While the highlight tones were severely compressed, these tones simply bunched up at the end of the histogram -- they were not clipped.
Noise is a major problem when enhancing shadow detail with Curves. Curves enhances shadow detail by amplifying the signal in the shadows. Unfortunately, the noise is amplified along with the signal. Since the SNR is so poor in the shadows, this can result in poor, shadow, image quality. This is shown in Figures 7 -- 9.
Figure 7 shows an image with a lot of shadow and dark tones. While it may not be evident from this small figure, there is a lot of noise lurking in the shadows and darker tones due to the poor SNR in those areas. However, when a curve was applied to the image to lighten the dark areas, the noise problem became obvious. Figure 8 shows a crop from the image that was taken after Curves was used to lighten the image. As can be seen, noise is a major problem.
On the other hand, noise is generally not a problem in the highlights. This is because of the relatively high SNR in the highlights. This can be seen in Figure 9, which shows a light area from the image in Figure 7 after the application of the same curve used in the shadows. As can be seen, Figure 9 shows far less noise in the highlights.
The last issue of concern is the data that is clipped by the tonal curves at the time of conversion (either in the camera or a raw converter). Unfortunately, this data has already been lost in the image that is being edited (that data is still contained in the original raw file if the image was shot in raw). Curves can not reclaim that data. Therefore, the shadow and highlight detail that was contained in that data is also lost.
While Curves is certainly easy, it has a number of limitations. Curves is limited in what it can do in the shadows due to the small number of levels and the poor SNR in the shadows. Furthermore, Curves suffers from tonal stretching and compression and it is unable to reclaim the data lost due to the application of tonal curves.
The Exposure tool (found in raw converters) and the Brightness/Contrast tool (found in photo editing programs) perform a similar function. Both serve to lighten or darken an image (both of these tools will be referred to as Exposure/Brightness for simplicity). As with Curves, Exposure/Brightness is an easy tool to use and is a fairly popular tool for making tonal adjustments.
Figure 10 shows an Exposure tool used to enhance shadow detail. Figure 11 shows a Brightness/Contrast tool used to enhance highlight detail.
Exposure/Brightness has the same limitation as Curves when it comes to the tonal levels. In the shadows, Brightness/Contrast is limited to the relatively small number of levels that the shadows contain. Thus, Exposure/Brightness is limited with respect to what it can do to enhance the shadows since the tool has to deal with a limited amount of information.
Again, in the highlights, the larger number of levels provides more information to deal with. Consequently, Exposure/Brightness can produce better results than in the shadows.
As always, raw files provide many more levels than JPEG. This will allow for better results when using Exposure/Brightness. This is especially true in the shadows where the problem of the limited number of tonal levels is the greatest.
One big difference between Exposure/Brightness and Curves is that Exposure/Brightness does not stretch or compress the tones. Rather, Exposure/Brightness is a linear tool. This means that the tool increases or decreases each tone by the exact same amount. This has both good and bad aspects for the photographer. On the good side, since the tones are not stretched or compressed, Exposure/Contrast does not increase the distance between the tones. This is shown in Figures 12 and 13. Figure 12 shows a histogram of a JPEG image that has not been edited. The histogram is smooth with no tonal gaps or compression. Figure 13 shows the histogram after Exposure/Brightness was used to lighten the image. This histogram also is very smooth and shows no tonal gaps or compression.
The benefits due to a lack of tonal stretching and compression with Exposure/Brightness is twofold. First, since Exposure/Brightness does not stretch the tones, the use of the tool will not cause banding. Thus, if an image shows banding due to the use of Curves, one option would be to try the Exposure/Brightness tool to see if the banding is eliminated (it is possible that the banding was partly due to tools other than Curves, so switching to Exposure/Brightness may not solve the entire problem). Second, since Exposure/Brightness does not compress the tones, the use of the tool will not cause a loss of tonal levels due to quantization error. In short, Exposure/Brightness does not cause the type of image degradation that Curves does.
However, not all is rosy with the use of Exposure/Brightness. Since there is no stretching of the tones, the use of Exposure/Brightness does not increase the contrast (which helps to make the detail more noticeable), as does Curves. This lack of contrast boost may make Exposure/Brightness somewhat less effective at enhancing shadow and highlight detail. This can be seen in Figure 14. Rather than stretching out the darker tones to cover the entire range of the shadows (thereby increasing the contrast), the entire histogram has been moved to the right. This created a huge gap on the left side of the histogram (this histogram shows no true blacks left after enhancement of the shadows).
Exposure/Brightness also causes clipping. This can be seen by examining Figures 14 and 15 again. Figure 14 shows that the right side of this histogram has been clipped after use of Exposure/Brightness. This clipping causes a total loss of detail in the clipped area. If the clipping occurs in the shadows, the darkest shadows will become blocked (they will go to pure back with no detail). If the clipping occurs in the highlights, the highlights will be blown out (they will go to pure white with no detail).
One way to eliminate the problem of clipping with Exposure/Brightness is the use of masks or the Blend If controls. Masks and the Blend If controls can be used to protect the areas that are not being enhanced from the clipping. For example, if Exposure/Brightness is used to enhance the shadows, a mask could be used to protect the highlights from the Exposure/Brightness adjustment -- thus, eliminating the clipping of the highlights.
Noise is also a problem with Exposure/Highlight. When lightening the shadows, Exposure/Highlight also makes the noise more visible. This tool simply can not overcome the poor SNR that exists in the shadows. However, noise in the shadows may be a bit less of a problem than with Curves. This is probably because the Exposure/Brightness does not increase the contrast. Figure 16 shows the same section of the image that had the dark tones enhanced by Curves in Figure 8. While the noise is still evident, is less noticeable than that in Figure 8.
The better SNR in the highlights allows for highlight enhancement with Exposure/Brightness without significant noise problems under most conditions.
To finish this tool, the issue of the data lost when a tonal curve is applied to an image needs to be considered. Unfortunately, Exposure/Brightness is not capable of reclaiming this data or the shadow and highlight detail inherent in it.
While Exposure/Brightness eliminates the problem with tonal stretching and compression that occurs when Curves are used, it replaces it with a clipping problem. Furthermore, Exposure/Brightness also suffers from the dearth of tonal levels and poor SNR in the shadows. Lastly, it is unable to reclaim the data last to the tonal curves.
Shadow/Highlight was designed specifically to enhance shadows and highlights. As such, it has some features that the Curves and Exposure/Brightness tools do not. This yields some advantages for photographers, but along with those advantages come some liabilities.
Figure 17 shows the Shadow/Highlight Tool. The first thing that should be noticed is that Shadow/Highlight has three sets of controls. The first set handles the shadows. The second set handles the highlights. The third set handles color correction, midtone contrast, and the Black Clip and White Clip. This article is primarily concerned with the first two sets of controls.
There are three controls used to adjust the shadows. Amount determines how much the shadows are lightened. Tonal Width determines how wide of a tonal range is affected. If the Shadow Tonal Width is set to zero, only the darkest shadows will be lightened. As the Shadow Tonal Width is increased, more of the shadow areas are affected by the adjustments (successively adding shadows from the darker to lighter shadows). Radius smoothes the transition from the areas that are affected by the editing to those areas that are not.
The Highlight controls are basically the same. Amount determines how much the highlights are darkened. Tonal width determines how wide of a tonal range is affected. A Highlight Tonal Width of zero causes only the brightest highlights to be affected by the adjustments. As the Highlight Tonal Width is increased, more of the highlight areas are included in the adjustments (successively adding highlights from the brighter to the less bright highlights). Again, Radius smoothes the transition from the areas that are affected by the editing to those areas that are not.
The big advantage here is that the corrections can easily be limited to specific tonal areas. For example, the highlights can be adjusted without affecting the midtones and shadows.
Again, as with the previous two tools, the first drawback of Shadow/Highlight is the limitation with respect to the small number of tonal levels in the shadows. The amount and quality of the detail that can be extracted from the shadows will be constrained by this scarcity of levels.
As before, the story in the highlights is much better. Since, there is more detail to work with, the results of the editing will be much better than in the shadows.
When working with Shadow/Highlight, the use of raw files will improve the efforts to enhance shadow and highlight detail as compared to the results achieved when using JPEG.
When one first encounters Shadow/Highlight, it is natural to assume that Shadow/Highlight works like a sophisticated Curves. Consequently, it might be expected to see significant stretching and compressing of the tones as was seen with Curves. However, a look at Figures 18 and 19 tell a different story. Figure 18 shows a histogram from an image that has not been edited. The histogram is smooth with no tonal gaps or compression. Figure 19 shows the histogram after Shadow/Highlight was used to bring out the shadow detail. This histogram shows little or no stretching of the shadow tones. Further examination of the histogram shows that the shape of the histogram in the midtone and highlight areas has remained relatively unaffected by the editing. In other words, the highlights have not been compressed.
Shadow/Highlight does not clip any of the tones. This can be seen in both 19 and 21. Both Figures clearly show that the tones at the ends of the histogram have been preserved.
With respect to Noise, Shadow/Highlight suffers from noise problems in the shadows. The poor SNR in the shadows makes it difficult to enhance the detail without concurrently magnifying the noise.
Figure 22 shows the same image that was used to demonstrate shadow noise problems for Curves and Exposure/Brightness. As with those tools, Figure 22 clearly shows a noise problem in the shadows when Shadow/Highlight is used for shadow enhancement.
With respect to the data that is lost when a tonal curve is applied, Shadow/Highlight is not capable of reclaiming that data.
Shadow/Highlight has little tonal stretching and compression and avoids clipping, but it has the problems of limited shadow levels, poor shadow SNR, and the inability to access the data beyond the tonal curves.
There are two additional issues when using Shadow/Highlight. First, Shadow/Highlight is not a layer tool. In other words, the tool does not create its own layer, as can be done with Curves or Brightness/Contrast. Shadow/Highlight can only be applied to an existing layer. Thus, it is preferable to create a new layer (either duplicate the Background layer or add a new layer that has the effects of the lower layers merged into it) and apply Shadow/Highlight to the new layer. Second, Shadow/Highlight has more of a learning curve than Curves or Exposure/Brightness. Inexperienced photographers can encounter problems with halos, unintended sharpening, and loss of detail with Shadow/Highlight if is used in an unskilled manner.