Or better, the loss of it?
If I take a picture of a uniformly white wall, I don't expect contrast at all and that's ok.
But sometimes I take photos of a a scene where there are light areas, dark shadow, lot of different colors...and I end up with a "lovely" grey covered image, with a typical histogram all by a side; nothing I can't recover by working the raw, but...I'd like to know what makes or remove the contrast.
My camera is not top notch, and moreover my usual lens is not that good so ok, that can make things worst.
But, even considering it...where contrast exactly come from? Is it really just the "difference" between the blacks and the whites? So why my camera sometimes take a "normal" scene with poor contrast? Is it the scattering in the air? How the lens can physically "lose" contrast?
I know my eyes can deceive me, for they take lots of different pictures and my brain (or whatever else is inside my head, not sure) compose them together, but I thought this works for high dynamic range situation...is it just that my eyes are really that good and use the same technique to improve the contrast?