Brightness / Darkness and Contrast


This adjustment is fairly self explanatory. However there are several different ways to accomplish these adjustments. The most simple is the Darkness to  Brightness and Contrast slider type adjustments that you find in most imaging programs. See an example on the left.

Adjusting brightness and contrast can also be done with a "levels" tool or a "curves" tool. These two tools are found in most INTERMEDIATE level imaging programs.

The levels tool or curves tool approach to tonal range adjustment allows much more precise control over the tonal range of your image. Tonal range within all digital images is represented graphically by assigning 0 to black and 255 to white. The equivalent of middle gray is represented at about 128. The tools discussed below are very powerful and I've only outlined some of their very basic capabilities.

With a levels tool you can adjust the dark tones, medium tones and light areas of the image separately. In the example on the left the dark graph area represents the distribution of tonal range of the pixels in your digital image. This type of graph is called a Histogram. Higher peaks represent a lot of pixels in that particular numerical tonal range. low peaks or no dark graph area within the graph window means few to no pixels are in that tonal range (0 - 255). The small triangle at the bottom left of the graph is the "black point" adjustment for dark tones. The gray triangle in the bottom center of the graph is the "midpoint" adjustment for midtone areas of your image and the white triangle at the bottom right of the graph is the "white point" or highlight adjustment slider. Moving these sliders "remaps" the existing pixel values in your image to the numerical value represented by the new position of the slider after the adjustment.

This particular histogram represents a fairly well balanced tonal range. Adjust white and black point first and mid point last because adjusting the white and black slider automatically resets the mid point slider to equalize your new tonal range. As an example of a levels adjustment, if this image was from a very dark photo, the histogram graph would be bunched up to the left with few if any pixels above 128. We could move the white point slider over to the beginning of the actual graph and expand the tonal range to contain a full range of tonal values from 0 - 255. See a detailed article on Interpreting RGB Histograms.



With a curves tool you can adjust any part of the tonal range of your image separately. This is accomplished by setting points on the graph line and "bending" the line by dragging these points. The bottom left of the graph's line represents black or "0" and the top right represents white or "255". The middle of the line is the equivalent of middle gray at "128". Setting a point in the middle of the graph and dragging that point up will raise the midtones in your image while simultaneously and automatically adjusting the tones below and above to blend with your new midtone setting. In the example on the left I have set three points. I've anchored the midtone area and set one point below midtone and one point above midtone. I've raised the upper point to lighten high tone areas and lowered the bottom point to darken the areas below midtones. This is a standard method for increasing contrast in an image.