Intermediate Exposure Adjustments

Exposure, brightness, lightness, and luminosity all describe the same basic concept, how light or dark an image appears. However, in real-world application, they’re not the same.

Exposure takes place in the camera, and once it happens, it can never really be adjusted. That said, most post processing applications have an exposure adjustment to simulate the effects of exposure. Exposure is more or less “linear”; the more light you let into the camera, the whiter the whites will become until they “wash out” and all detail is lost. However, film also has response curve. Therefore, adjusting exposure is tricky in post processing.

Brightness refers not only to luminosity, but to color as well. An image is considered to “bright” when it’s both light and has clean, vibrant colors, in other words good color saturation. The luminosity level may be perfect, but if the colors do not have adequate saturation, the image will appear dull.

Lightness refers only to luminosity, hence the “Lightness” channel in the L*a*b* color space. When the color is right, we want to be able to control only the lightness, and this is not something that can be done by adjusting exposure.

Luminosity generally refers only to lightness, and not to color. The Luminosity blend mode in Photoshop restricts an adjustment to the information that would appear in the Lightness channel of a L*a*b* image. However, used descriptively it can also refer to brightness. A “luminous” image is one that is both light and colorful.

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Curves
The advantage of Curves is that you can use them to adjust the brightness or lightness of values at up to fourteen points in a infinity number of locations along the tone curve, while preserving the white and black points. This makes it both an exposure and a contrast tool, which can be used to adjust both exposure and gamma.

Choose any point along the curve and drag up to brighten/lighten the image or down to subdue/darken the image in that particular tonal range without affect the white or black points. (If you’re in the Pigment/Ink mode the curve is moved in the opposite direction).