Just Say "No" to Brightness and Contrast in Photoshop
Speaking in terms of 8-bits per channel, there are 256 possible brightness values available for use, ranging from black on one end to white on the other. Many times, images straight out of the camera or from the scanner lack contrast or are underexposed and don't make use of this full range. New Photoshop users often try to remedy these problems by using the Brightness and Contrast controls on the Image Adjustments menu. Resist the temptation though. Just say "no" to Brightness and Contrast.
To illustrate why, let's start with a sample photo that is underexposed. The image and its histogram are shown below on the left. Given that the lupine leaves should be medium toned green, let's increase the Brightness until the histogram comes out centered. On this particular image, it'll take a fair bit of increase, but it can be done. What we end up with is shown in the middle image. While it's no longer underexposed, it is still significantly lacking in contrast and looks quite washed out. The histogram for this version confirms things are pretty bad. Note that it still has basically the same shape as the original histogram and that increasing Brightness has merely shifted the entire histogram towards the right. While this may not be what we had hoped for, it is what we asked for. Brightness works by adding whatever we set the control at to the value of every pixel in the image. If the image did have values ranging only from 0 to 128 and we increase brightness by 64, the resulting image will now have values 64 to 192 (0 plus 64 and 128 plus 64). Everything is increased evenly, in a linear fashion. Similarly, decreasing Brightness just shifts everything in lock step towards the left. Increasing Brightness is guaranteed to leave a gap on the left hand end and decreasing it will always leave an empty space on the right of the histogram. Nothing but simple arithmetic at work here.
If we try to fix this by using the Contrast slider, the histogram is stretched evenly towards both left and right. The water droplet highlights in our sample though were actually not too bad to begin with so we really need more of a fix on the left than on the right. If we stretch the contrast far enough on the shadow end to fix it, we will force the highlights too far on the right, clipping them at the edge of the histogram. In fact, everything in the image now comes out looking quite harsh and unnatural as you can see in the image on the right. Like Brightness, Contrast works equally over the entire range of image brightness.
To put it bluntly, it's just not possible to do a good job with Brightness and Contrast. They both work far too simply and offer far too little control to be very useful. That may leave you wondering what to use instead, and as with most everything else in Photoshop, Adobe has provided numerous good answers that we'll take a look at starting next week with Levels.
Frankly, it would be better for all concerned if Adobe would drop Brightness/Contrast from the product. Until they do, you're better off simply ignoring them.
Update 10/14/2007 - Things have changed with the release of Photoshop CS3. Click here to find out how.