Gamma correction involves brightening or darkening mid-tones.
While simply adjusting brightness or contrast affects an entire image and may brighten dark areas too much or make bright spots too dark, gamma correction preserves an image's natural patches of shade and light.
The above picture's mid-tones are too dark.
Here, the picture's mid-tones are balanced.
The mid-tones of this picture are too bright.
Zdjęcie zbalansowane w tonach średnich.
Since gamma correction only affects mid-tones, the natural brightness levels of black and white are preserved.