Adjusting Brightness
After you have achieved good contrast, your image might look too bright or dark. The middle slider in the Levels dialog box can fix that. (Techies love to call this slider the Gamma setting, but we plain folks call it the midpoint.) If you move the middle slider to the left, the image will become brighter without messing up the dark areas of your image. Black areas will stay nice and black. Or you can move the middle slider to the right to darken the image without messing up the bright areas of the image. White areas will stay bright white (Figure 5.23). This is the one setting that is a personal choice. I can't tell you how bright or dark your image should be.
Figure 5.23. Effects of the middle slider.
If you want to know what this adjustment is doing, just look directly below the middle slider; the shade of gray there will become 50% gray. Moving it to the left will brighten your image because you'll be shifting what used to be a dark shade of gray to 50% gray. Moving the middle slider to the right will darken your image as you shift a bright shade to 50% gray. If you look at an updated histogram of the image, it will look like you stretched out a Slinky, then grabbed one side and pulled it to the middle (Figure 5.24). Some bars will get scrunched (is that a technical term?) together, while others get spread apart.
Figure 5.24. The adjustment shown on the left results in the histogram shown on the right.
 | Spikes that show up after an image has been adjusted with Levels do not indicate noise. It's as if you took your trusty Slinky and tried to squish it down to a centimeter wide. Something would have to budge. The only way I can do it is to bend the Slinky into a V shape where the loops start piling up, one on top of the other. Otherwise, the loops just line up in a nice row and limit how much I can compress the Slinky. Well, the same thing happens with the histogram. Let's say you try to squish 20 bars into a space that is only 15 pixels wide on the histogram. Five of the bars have to disappear. They are going to just pile on top of the bars next to them and make those bars about twice as tall. When this happens, you get evenly spaced spikes across part of the histogram. |