Photos are rarely taken in ideal conditions. When low or too bright light is available, pictures may appear dark or overexposed. There are several ways to correct this.

This picture was taken without a flash. It is so dark the girl and her baby brother can barely be seen. One way of correcting is to use the brightness and contrast sliders. This will correct the extremes of brightness but neglects the middle tones, as in the picture below.

Using the levels histogram, shown below, allows us to choose a white, gray and black point with set point droppers. We can correct for middle tones using the midtone slider. This gives us more control in correcting for brightness and contrast, and the result is a more even distribution of light and dark. We can then apply an Unsharp Mask to correct for any blotchiness that might result. (See the page on Filters & Effects.)

This picture was corrected using Levels and the Unsharp Mask filter. You can see the difference in color depth and clarity.