Adjusting contrast

[Redbriars]

When I take an image (a jpeg in colour, for example) into the editing mode of my software (ACDSee) and click on the exposure tool, I am offered three sliders: adjust exposure, adjust contrast, and adjust fill light.

Would someone please explain what goes on at a pixel level with these commands? I seem to think that each pixel has 256 levels of brightness. (is this so?) The help file says that increasing the fill light brightens the darker pixels in the image. What do the other two commands do at a pixel level?

TIA . . . Alan

[dem]

In a nutshell, increasing exposure makes all pixels brighter, whereas increasing contrast makes light pixels brighter and dark pixels darker.

[Manfred M]

I seem to think that each pixel has 256 levels of brightness. (is this so?)

That is true only for an 8-bit image. A 16-bit image has 65,536 levels of brightness for each pixel.

I'm not sure what the "adjust fill light" slider does, as I don't use ACDSee. My guess is that it is similar to what the "shadow" slider does in Camera Raw / Lightroom, which is to lighten the shadow detail (data that has low numbers, i.e. close to 0), while not touching the highlights (data with high values, approaching 255).