There are two types of contrast that can be used to make images more interesting: color contrast and tonal contrast. But for now I only want to talk about is tonal contrast. And specifically the two types of tonal contrast: overall contrast and midtone contrast, and how they can be modified during post processing.
The overall contrast of a print is the difference between the brightest significant highlight and the darkest significant shadow tone. If a scene is captured in subdued light, chances are the histogram for the captured image does not extend out to both ends. Many images with this type of histogram will look better if the tonal range is stretched so the number of tones in the image is increased. This stretching can be accomplished by setting a black and a white point for the image. When setting the white point the histogram is stretched to the right; usually, until the graph is just about to touch the right of the histogram. And when the black point is set the histogram is stretched to the left. The best place to set the black and white points is during raw conversion. (Perhaps in a future post I’ll explain why it’s best to do it during raw conversion as opposed to doing it Photoshop.) In Adobe Camera Raw (ACR) or Lightroom’s Develop Module the white point is set with the Exposure slider and and the black point is set with the Blacks slides. They both also have a clipping preview to assist in setting the black and white points.
To access the clipping preview hold down the Alt key when dragging any of these sliders: Exposure, Recovery, and Blacks. Assuming that the histogram is not already touching the end points, when the Alt key is depressed and the Exposure slider is activated the display will go completely black. Slowly drag the Exposure slider right until some non black areas start to appear. Pure white means those are blown pixels. Colored pixels mean that one or two RGB channels are maxed out. Now drag the Exposure slide back a bit to the left until the screen goes pure black–no blown out pixels. Setting the black point works analogously but the initial display is pure white.
The screen shot above is part of the ACR window with highlight clipping active. (If I’d have thought about the blog background being black I would have done shadow clipping so the screen would have been white. Here the black of the screen shot looks like part of the background.) Here I’ve moved the Exposure slider a little too far and some pixels are clipped. The white is where all three RGB channels are clipping. The red is where only the red channel is maxed out. And so on.
Just to recap, by setting a black and white point you are setting the overall contrast–the difference between the brightest bight and the darkest shadow. Or to say it another way, for the image you have set the maximum number of tone that exit in the image. Now lets move on to midtone contrast.
Midtone contrast is manipulated with curves. We increase midtone contrast while decreasing highlight and shadow contrast. I talked briefly about it in Curves Replaces Levels–Part 2. By using an S-Curve we change the distribution of the overall contrast. This is a screen shot of ACR Point Curve. Here I raised the 1/4 tones just a tad. Then I drug the darks down a bit. This gave me the midtone contrast pop I was looking for but it also darkened the image a bit too much so place a point to force the curve to cross the base line deep in the midtones but a bit before the 3/4 tones.
By brightening the brights and darkening the shadows I increased contrast. But this time it was midtone contrast. The overall contrast is set with the black and white points. With curves we brighten the brights and darken the shadows without affecting the brightest highlights and the darkes shadows. The portions of the curve that are steeper than the baseline have increased contrast. In portions of the curve that are flatter than the base line, the contrast is decreased. So the midtones have increased contrast while the highlights and the shadows have decreased contrast. We have used the S-Curve to redistribute the contrast that was available.
If you stuck through all this you deserve to see the image used for the screen shots. Here it is:
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