An histogram is a graphic representation of the tone values of your image. It show the amount of each tone of a brightness from black (0% brightness) to white (100% brightness).
If the histogram reach either edge, it indicate loss of detail, also called clipping.
– Highlight clipping : right edge, areas completely white, absence of detail (called blown out)
– Shadow clipping : left edge, areas completely black, absence of details (called clipped)
It can be fixed by adjusting the exposure (use exposure compensation). Always remember that it dependes on the scene (ex : sun, white background in studio…) so it could be normal to have a clipping.
Histogram for a dark subject (black cat)
Histogram for a light subject (with mostly light tones in the scene and few dark areas)
A gap between the edge and the histogram indicates a loss of details and could be easily correct by adjusting exposure
EXPOSURE COMPENSATION (+/- button)
To keep shadow detail, make images brighter by dialing in positive exposure compensation value (+0.3 or +0.7, for example)
to save highlight detail, make images darker by dialing negative exposure compensation value (-0.3 or -0.7, for example).
THE BLINKIES (or ‘Highlight Mode’)
Most SLR cameras have a setting called “highlight warning”. It will make any overexposed highlights “flash” or blink when you preview your images on your camera screen.