…is the most important control after focus, if you use your camaera’s semi-automatic modes.
What does it do? It makes the picture darker and light.er But how? Does it change the pixels? Adjust the ISO? Change aperture? Do processing in the chip? What?
Actually it is very simple.
You use exposure compensation (the +/- button on your camera) only in modes where the camera is already adjusting something.
If you are in aperture mode (A/Av), the camera constantly adjusts the shutter speed to match the light. If you are in shutter speed mode (S/Tv), the camera adjusts the aperture. In Program mode (P), the camera adjusts either/both.
All you are saying with +/1 is “I want you to do that as usual, but to do it slightly differently to how you’d normally set it. + means do what you do but make it brighter than you’d normally do; – means do what you do but make it darker than you’d normally do (like in the picture below).
So in Av/A mode, it adjusts the time as usual but to a slightly different value. In Tv/S mode, ditto for the aperture. In P mode, either.
No magic, then.