Today’s Question: What is the difference between an exposure adjustment and a brightness adjustment?
Tim’s Quick Answer: The answer here depends on the software being used to apply the adjustment. In general an exposure adjustment applies a tonal adjustment with a value based on stops of light, while a brightness adjustment is based on a percentage. In addition, the specific algorithm used for each type of tonal adjustment will vary (sometimes significantly).
More Detail: First off, I think it is important to keep in mind that in general we have a lot of terms that refer to overall tonal levels in a photo. This includes exposure, brightness, tonality, lightness, and luminance, among other terms. In many respects the terms “exposure” and “brightness refer to the same thing. Both terms refer to overall tonal values (another word for the same basic thing). However, these terms are often used in different contexts, such as in the camera versus when adjusting the photo after the capture.
In general I would say that the term exposure is most often used to refer to the tonal levels in the camera at the time of capture. In turn, an exposure adjustment in post-processing software is generally aimed at mimicking what might be accomplished in the camera by adjusting the exposure. Thus, the Exposure slider (such as that in Lightroom and Adobe Camera Raw) will use a scale based on exposure value (EV), which is measured in stops of light.
A brightness adjustment also obviously adjusts the overall tonal values for an image, though with an adjustment typically measured as an overall percentage rather than exposure values.
Of course, behind the scenes there can also be significant differences between adjustments for tonality that have a different name. In many cases, for example, you may find that an exposure adjustment in post-processing applies an effect that is much more sophisticated than a simple linear increase or decrease in brightness values. With a linear adjustment you are affecting all tonal values in the image equally, while with a more sophisticated adjustment you may have a variable effect on different values, possibly even excluding certain tonal values from the adjustment altogether.
The challenge here is that each different software application might take a different approach to overall tonal adjustments with different names. The method used for each adjustment might even change with different versions of the software. For example, a handful of versions of Photoshop ago, the Brightness slider for the Brightness/Contrast adjustment changed from a linear adjustment to a more sophisticated adjustment that helped prevent clipping in the extreme tonal values.
What that really means is that there isn’t a universal definition for each of the terms that refer to tonality in the context of software used to adjust tonality in an image.