From BlenderWiki

Exposure and Range

Description

Exposure and Range are similar to the “Color Curves” tool in Gimp or Photoshop.

Previously Blender clipped color straight with “1.0” (or 255) when it exceeded the possible RGB space. This caused ugly banding and overblown highlights when light overflowed (An overexposed teapot).

Using an exponential correction formula, this now can be nicely corrected.

Options

Exp

The exponential curvature, with 0.0 being linear, and 1.0 being curved.

Range

The range of input colors that are mapped to visible colors (0.0 – 1.0).

So without Exposure we will get a linear correction of all color values:

1. Range > 1.0: the picture will become darker; with Range = 2.0, a color value of 1.0 (the brightest by default) will be clipped to 0.5 (half bright) (Range: 2.0).
2. Range < 1.0: the picture will become brighter; with Range = 0.5, a color value of 0.5 (half bright by default) will be clipped to 1.0 (the brightest) (Range: 0.5).

Examples

With a linear correction every color value will get changed, which is probably not what we want. Exposure brightens the darker pixels, so that the darker parts of the image won’t be changed at all (Range: 2.0, Exp: 0.3).

An overexposed teapot.	Range: 2.0.
Range: 0.5.	Range: 2.0, Exposure: 0.3.

Hints

Try and find the best Range value, so that overexposed parts are just not too bright. Now turn up the Exp value, until the overall brightness of the image is satisfying. This is especially useful with area lamps.