Basic 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.

So while this tutorial is named “Basic Exposure Adjustments” we will also talk about adjusting the brightness, lightness and gamma of an image.

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Brightness/Contrast
Perhaps the simplest and most powerful tool for adjusting exposure is the Brightness/Contrast panel. Unfortunately, it’s also the most misunderstood amongst professional color retouchers.

In earlier versions of Photoshop, B/C was strictly a linear adjustment, much like adjusting exposure in a camera. However, in recent versions, this has been changed to a gamma adjustment, with a check box to revert back to the “legacy” linear algorithm. This combined with the fact that we can use it with the Luminosity mode and the Blend If sliders makes it much more powerful that it initially appears.

Drag the Brightness slider to the right to brighten or lighten the image or to the left to subdue or darken the image. The Contrast slider makes the adjustment behave similar to Curves, and is discussed in Intermediate Contrast Adjustments.

Keep in mind that color saturation is naturally dependent on luminosity, so beyond a certain point, colors may appear washed out. If this occurs, use the Luminosity mode to regain vibrance.

Exposure or “brightness” of the image, can be adjusted with Brightness/Contrast by checking the “Legacy” box. This will create a linear adjustment that shifts all the values in the image toward the highlights. When doing this, we have to be sure not to shift them so far that the highlights wash out. Once we reach that point, we can go no further.

Midtone Gamma is adjusted when the exposure is correct (the highlights of the image are matched to the “white point” of the color space) but the image needs to be lightened or brightened. To adjust the gamma, be sure to uncheck the “Legacy” box. If you want only to lighten the image, and not brighten it (affect color), then use it with the Luminosity blend mode. If you are adjusting a CMYK image, the Luminosity mode must usually always be used in order to maintain color balance.

Lightness can be adjusted by using the Luminosity blend mode with the Legacy box unchecked (gamma adjustment) or checked (linear adjustment).

Brightness can be adjusted (gamma or linear) by using the Normal blend mode. For stronger brightness adjustment with more control over hue, saturation and contrast see Advanced Exposure Adjustments.

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Levels
If you need to set the white and black points (“endpoints”) of an image, Levels can be used to adjust brightness or lightness at the same time. Drag the midtone gamma slider (gray) to the left to increase the gamma (lighten the image) or to the right to decrease the gamma (darken the image). Levels can be used with the Normal or Luminosity modes, but CMYK image almost always require Luminosity to maintain correct color balance.

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Exposure
The exposure sliders found in Adobe Camera Raw, Adobe Lightroom and Apple Aperture all use algorithms specifically designed to simulate the effects of exposure adjustment within the camera. They are calibrated in stops rather than arbitrary numbers, and are fairly accurate, although their effects will vary widely between applications. They are arguably the best means of adjusting exposure.