Adobe Photoshop CS5 for Photographers - Martin Evening [146]
2.
One way you can use Levels to improve the tonal contrast is to simply look at the histogram in the Levels adjustment and drag the Input sliders inwards until they meet either end of the histogram. However, you can get a better idea of where to set the endpoints by enabling the Threshold display mode. To do this, I held down the key as I dragged the shadow Input levels slider inwards. The preview now showed a threshold mode view that enabled me to discern more easily where the darkest shadows were in the picture. The threshold view you see here is rather extreme, so I would want to back off from this setting, otherwise the shadows would be too clipped. The general idea here is that the black clipping in the Threshold mode preview indicates the points at which the blackest blacks will be clipped.
3.
The same technique could also be applied adjusting the highlights. Here, I held down the key as I dragged the Highlight slider inwards. As I did this the Threshold display mode started off completely black and the lightest points in the image (where I might want to clip the highlights) appeared first as I dragged the highlights Input levels slider inwards. As in the previous example, this preview shows an extreme adjustment so I needed to ease off a bit and search for the lightest highlight point and then maybe reduce the clipping slightly since I didn't want to risk clipping any of the important (non-specular) highlight detail (see pages 180–181 for more about how to clip the highlights).
4.
Here is the final version, which shows what the image in Step 1 looked like after I had applied the Levels adjustment described here. The levels in the photograph were expanded to reveal a fuller range of tones and as you can see, the Histogram panel view now confirms this.
Curves adjustment layers
Any image adjustment that can be done using Levels can also be done using Curves, except with Curves you can accurately control the tonal balance and contrast of the master composite image as well as the individual color channels. You can also target specific points on the tone curve and remap the pixel values to make them lighter or darker and adjust the contrast in that tonal area only.
Are Curves all you need?
I have become a firm believer in trying to make Photoshop as simple as possible. There are 22 different items listed in the Image Adjustments menu and of these I reckon that you can achieve almost all the image adjustments you need by using just Curves and Hue/Saturation. While Curves can replace the need for Levels, I do still like to use the Levels dialog, because it is nice and simple to work with, plus there are also a lot of tutorials out there that rely on the use of Levels. You are still likely to come across lots of suggestions on ways to tonally adjust images by various means. And you know what? In most cases these techniques can often be summarized with a single Curves adjustment. It's just that it can sometimes actually work out quicker (and feel a lot more intuitive) to use a slightly more convoluted route. In Photoshop there is often more than one way to achieve a particular effect, but to quote Fred Bunting: there are always those techniques that are ‘more interesting than relevant.’
As with all the other image adjustments, there are two ways you can work with Curves. There is the direct route (using the Image Adjustments menu) and the Adjustments panel method described here. In the case of Curves, the two layouts are quite different and I have chosen to concentrate on the Adjustments panel here first because I believe this method is more useful for general image editing. However, the direct Curves dialog does offer some unique legacy features that are still worth mentioning.
Figure 5.25 shows the Adjustments panel displaying the Curves controls, where the default RGB units are