Adobe Photoshop CS5 for Photographers - Martin Evening [150]
Figure 5.33 Setting the output levels to something other than zero is not something you would normally want to do, except for those times where you specifically want to dull down the tonal range of an image. In the example shown here, the left section shows standard optimized levels, the middle section shows the same image with reduced highlight output levels and the right section shows the image with reduced shadow output levels.
Using Curves to improve contrast
So far I have shown how a Levels or Curves adjustment can be used to expand the levels in order to achieve an image in which the tones have been fully optimized. However, there are also times where you may wish to modify the contrast, but without clipping the levels at either end of the histogram. This is where Curves adjustments come into their own, because Curves allow you to modify the image tonal contrast with fine-tuned precision.
Figure 5.36, Figure 5.37 and Figure 5.38 show three examples of how you can use a Curves adjustment to manipulate the contrast. In Figure 5.34, the shadows and highlights had been optimized, but the curve was left as a straight line between these two points. In this example a Curves adjustment was used to optimize the image contrast in exactly the same way as you could do using Levels. In Figure 5.35, two curve points were added to create a steep ‘S’ shape curve. By steepening the curve you can increase the contrast in the tones that reside in that portion of the curve. In this example, the shadow and highlight points were optimized and more contrast was added to the midtones. In Figure 5.36, two curve points were added to create a shallow ‘S’ shape curve. In this example you will notice that although the shadows and highlights have again been optimized, there is now a decrease in contrast in the midtone areas. This occurred where the curve was at its shallowest.
Figure 5.34 This view shows an image where the Curves adjustment simply optimized the input shadows and highlights and no further adjustments were applied to the curve.
Figure 5.35 In this example, the contrast was increased by adding two extra curve points to create the ‘S’-shaped curve shown here.
Figure 5.36 In this example I decreased the contrast by adding two curve points to create an ‘S’-shaped curve that flowed in the opposite direction to the one shown in Figure 5.35
When you first start working Curves you should aim to add just a few points at a time. As you develop your Photoshop skills you can try adding more points, but as you do so, take care to maintain a nice smooth curve shape. Adding more points (especially if they are set too close together) can cause sharp kinks in the curve which in turn can produce an undesirable ‘solarized’ look, or possibly some banding.
Curves luminance and saturation
A Normal mode Levels or Curves adjustment will adjust both the luminance and the color in an image. This means that as you steepen the curve to increase the contrast, you will at the same time increase both the luminance and color contrast. It can therefore be handy to filter a curves adjustment by choosing a Luminosity blend mode to target the luminance contrast only, or a Color blend mode to target the color contrast only (see pages 330–331).
Luminosity and Color blending modes
One of the problems you may encounter when applying Normal mode tone adjustments is that as you use Levels or Curves to adjust the tonal balance of a picture, the adjustments you make simultaneously affect the color saturation as well. In some instances this might be a desirable outcome. For example, whenever you start off with a flat image that requires a major Levels or Curves adjustment, the process of optimizing the shadows and highlights will leave you with a nice bright-looking picture with increased contrast and colors that are more saturated. This can be considered to be a good thing, but if you are carrying out a careful tone adjustment