What is the difference between an exposure slider and a levels slider?

Question

One of the first things many postprocessing tutorials teach is to adjust the levels in a raster image editor, so the histogram uses the whole possible range. In my RAW editor (Darktable), there wasn't a levels setting, so if the histogram was too far in the middle, I used the exposure slider to stretch it to the left and right. I thought that, similar to levels, it just stretches the range in which there is information recorded, discarding the extreme parts. (Of course, I didn't stretch all pictures completely to both ends, but for simplicity's sake, let's say that we are talking about a motif for which it is the right thing to do).

Now, Darktable was upgraded to version 1.0, and got some new modules. One of them is a levels module. I assumed that using it to stretch the histogram will do the same as using the exposure module, but was curious to see if this is what really happens. I tried it, and noticed that it doesn't work this way.

This is what a picture can look like using both methods. The upper part was made with levels - the white part is darker, but has more details. The lower part was made with exposure. I have the feeling that it also affected the saturation of the greens, but I might be wrong.

These are the histograms. The first one is of the levels setting, the second one is of the exposure setting.

What is the real difference between the two settings? How do they work, to produce the different results, and why?

Answer 1

Exposure, as the name suggests, adjusts the overall exposure, just as if you had exposed the image a little more, or a little less, from the start. So if you move the exposure slider to the right, you add exposure across the entire histogram, shadows and highlights. You basically move the entire histogram to the right.

With levels, you set a black, white, and gray/midtone point. Effectively this stretches or shrinks the histogram, so you might move the dark pixels to the left, and the light ones to the right. Because you can move black and white points independently, you can adjust the overall contrast of the image (by moving those two points together or apart). With exposure, you are moving everything together, so you only lighten, or darken the image.

Answer 2

Another way to look at it...

The exposure slider is an algorithm-based adjustment. It doesn't uniformly move pixel intensity as you make adjustments. It also generally focuses on the mid-tones of an image.

The levels slider is all about pushing pixel intensity in a given direction wherever they fall in an image. It isn't an algorithm because your adjustments are the algorithm. You can make more precise changes in some respects but it's also harder to control. As mentioned, this affects contrast.