One of the most common edits performed on images is contrast enhancement. This often significantly improves the image impact. Most often, a photographer will adjust contrast with a tool such as Curves. This frequently produces the desired results. The key here is the word "frequently". Sometimes, Curves, or the other tools that modify contrast, may leave the image a bit lackluster. The contrast has been modified, but the image just doesn't seem to come alive. Now, why would this happen? Well, the issue here is that not all images require the same type of contrast adjustment. To produce the best results, a photographer needs to understand the types of contrast and how they affect an image.
The cactus flower image in Figure 1 is a good example of an image that needs some contrast enhancement. Probably, the first thing that is noticed is that the image lacks contrast.
That may have helped a little. A comparison of the image before curves (see Figure 4) and the image after Curves (see Figure 5) shows some improvement, but the image still looks kind of lackluster.
Okay, this is confusing. The image appeared to need some contrast enhancement. So, the contrast was increased, but the image still doesn’t really come alive.
Actually, the original statement that the image needed the contrast enhanced is correct. The problem is that the wrong type of contrast was enhanced. Now, you may be thinking, “The wrong type of contrast. What in the world is this all about?” Well, here’s the issue. There are actually two major types of contrast: large scale contrast and small scale contrast. The difference between large scale contrast and small scale contrast can be seen by taking a closer look at the contrast in the cactus flower image (without the Curves layer applied).
To get more information on the image tonality, the Info panel is accessed (see Figure 7). Notice that the first color readout is set up with the hue, saturation, and brightness color model. The brightness is the primary concern here. When the cursor is placed over the darker reds (see Figure 8), the brightness reading is 8%.
When the cursor is placed over the yellows (see Figure 10), the brightness reading is 88% (see Figure 9). This is a large difference. When the difference between these tones is changed, the large scale contrast is adjusted
Now, it is time to take a look at small scale contrast. Small scale contrast involves contrast between tones that are fairly close in tonality and proximity. With the cactus image, there is small scale contrast between some of the colors on the cactus needles. For instance, when the curser is placed over one of the reds on a single cactus needle (see Figure 12), the brightness reading is 41% (see Figure 11).
When the curser is placed over a slightly lighter part of the same needle (see Figure 14), the brightness reading is 53% (see Figure 13). This is a pretty small difference. When the difference between these tones is changed, the small scale contrast is adjusted
One might be tempted to think, “Why should we care about the difference between large scale and small scale contrast? We can adjust any contrast with Curves”. Well, there are problems with that approach. Comparing a close-up section of the cactus flower image without curves (see Figure 15) to the image with Curves (see Figure 16) is very informative.
It is obvious that Curves has increased the large scale contrast. This can be seen by comparing the yellow and red on the large flower bud. In Figure 15, the image looks dull and lacks contrast between the yellow and red areas on this bud. On the other hand, Figure 16 shows good contrast in this area. So, Curves has done a pretty good job of editing the large scale contrast.
However, an examination of the small scale contrast tells a different story. The small scale contrast between the tones on the cactus needles has increased only a small amount. The tones on the needles have darkened somewhat, but the contrast is still poor. Consequently, the needles still look lackluster. One might try adjusting Curves to get more small scale contrast. However, this would cause the large scale contrast to become too great -- making the image look over edited.
Now, one might be tempted to think that, if Curves were fine tuned enough, both the small scale and the large scale contrast could be optimized. On the other hand, such attempts will generally not be very successful. The reason for this is that, when a Curves edit is made, some tones are stretched out which increases contrast. Other tones are compressed which reduces contrast and results in a loss of tones. In simple language, this means that Curves can make certain tones look better but will degrade other tones. In short, if an attempt is made to use Curves to improve both the small scale and the large scale contrast, it will be very difficult to improve all the different tonal areas that need a contrast boost. For instance, some of the areas might need a small scale contrast boost of the light tones. Some of the areas might need a small scale contrast boost of the middle tones. Some of the areas might need a small scale contrast boost of the dark tones. In addition, the large scale contrast might need adjustment. Curves can not improve the small scale contrast in all of these different tonal areas as well as handle the large scale contrast. It can only improve some of the tonal areas at the expense of others. The bottom line is this. Curves works best on large scale contrast. It is not well adapted to editing small scale contrast. This is also true of the other tools that can be used to edit tones (e.g., Levels and Brightness/Contrast).
So, if Curves is not the best choice for small scale contrast, is there another technique that can be used to improve small scale contrast? As a matter of fact, there is such a technique. It uses a very commonly used tool, but it uses the tool in a way that many photographers have never used it before.
What is this mystery tool: Unsharp Mask (USM) applied differently than the way it is usually used.
To get started, the Contrast layer is selected and USM is launched by choosing Filter/Sharpen/Unsharp Mask. Up comes the USM dialog box (see Figure 17). Now, before any edits are made, it is important to understand what is really going on with USM. Usually, people think of USM as a tool for sharpening digital images. However, what USM really does is identify areas of high contrast. These areas are basically the edges between lighter and darker areas. Then, USM increases the contrast along those edges.
Next, it's important to understand how the three USM settings affect an image. The Amount setting controls the strength of the contrast along the edges. The higher the Amount setting, the stronger the contrast. In other words, the Amount setting determines how light USM makes the lighter pixels and how dark it makes the darker pixels along the edges.
At the bottom of the USM dialogue box is the Threshold setting (the Radius will be covered last). The Threshold determines how much tonal difference must exist along the edges before the contrast is increased. A setting of 0 means that almost any tonal difference will have the contrast increased. High threshold settings will result in the contrast along the edges being increased only if there are large tonal differences along the edges.
Last, there is the Radius. The Radius determines how wide of an area, around the edges, will have the contrast increased. A low Radius means that the contrast increase will be applied right next to the edge only. A larger radius applies the contrast increase further out from the edge.
So, why is the Radius covered last? The reason is that, when it comes to enhancing small scale contrast, the radius is the most important of the three settings. In fact, the Radius is the key to the entire technique.
As was previously mentioned, the Radius determines how wide of an area, around the edges, will be affected by the contrast increase. Usually, when USM is used, the radius is set to a fairly small value. Applying a radius of 1.5 and an Amount of 150 allows us to compare the unsharpened image (see Figure 18) to the image with USM applied (see Figure 19). Clearly, USM has sharpened the image.
Likely, that result didn't surprise anybody.
However, at a deeper level, what is seen is an image where the contrast has been significantly increased over a very narrow width around the edges. The human eye and brain interpret this type of contrast increase as an increase in sharpness.
To understand deeper what is going on, Figure 20 shows a close-up of the image in its unsharpened state. Figure 21 shows the close-up after USM was applied. First, it can clearly be seen that USM increased the sharpness of the edges. Second, when the small scale contrast on the needles is examined, it can be clearly seen that this application of USM did not have much of an affect on the small scale contrast. In addition, USM had no significant affect on the large scale contrast. So, the conclusion of this is that USM, with a small radius and a large amount, results in a sharpening of an image but has no real affect on either the small scale or large scale contrast.
Now, it's time to change the strategy a bit. The USM will be done over. However, this time, a large Radius value will be used. For this sharpening, a Radius of 20 and an Amount of 75 are used. Figures 22 and 23 allow a comparison of the image before and after this USM step.
Obviously, there is quite a bit of difference between these two images. A first reaction might be, “Okay, Figure 23 looks over done”. In fact, at this point in time, it is. On the other hand, something very important occurred here -- the small scale contrast has been increased significantly. This can be seen in the different shades of yellow on the flower buds and with the small scale contrast on the needles. In fact, the small scale contrast has been increased a bit too much and that is what makes it look over done.
There is reason to celebrate as we now have a method to enhance small scale contrast by using USM with a large radius.
Figures 24 and 25 compare the image with USM applied with a small Radius and a large Radius. The conclusion of this comparison should be that USM with a small radius and a large amount results in a sharpening of the image but has no real affect on either the small scale or large scale contrast. On the other hand, USM with a large radius results in a small scale contrast increase.
Figure 25 also appears to show some increase in large scale contrast. This is because a fairly large Amount was used when USM was applied. This was done to make it easier to see the effect on the small scale contrast. In actual practice, a much smaller Amount is used when applying large radius USM to enhance small scale contrast. This significantly decreases this technique's impact on large scale contrast.