Understanding Histograms

Histograms appear to be a bit of a mystery to many people, but they are very easy to understand and give a wealth of information on exposure. They show up in both digital cameras and in image processing programs, and are the same histogram.

The image on the above is underexposed (well... duh!) and the histogram leaves no doubt. The bulk of the graph is piled up against the left axis, and even the extreme highlight barely cross the centre. While it can be salvaged, quality will be strongly impacted. The peak is due to the large area of pavement in the picture which is OK. This image is certainly a bit on the dark side, but would make an excellent print. Little shadow detail is lost as can be seen by the slope on the left side as opposed to it being jammed against the axis. On the right, the highlights are not touching the right axis, indicating that all highlight detail is intact.
Pretty much ideal exposure. Only specular reflections are blown out, and shadows are rich and dark, but could easily be darkened a bit more by trimming the histogram a bit on the left to give the image a bit more snap. Overexposure. The graph is beginning to pile up against the right axis. Much highlight detail is lost and a quality print is impossible. When this condition is seen, exposure must be reduced. Image processing can not recover lost detail.
Interpretation of the image is rich in possibilities. Quite lovely to begin with, but here clipped both left and right to bring out the warmth of prairie sunshine.
Low contrast. A very comfortable histogram. The scene is low in contrast, the histogram is touching neither axis. No opaque shadows nor blocked highlights.
High contrast. The histogram is jammed against the left axis showing a loss of shadow detail, and climbing the right axis showing blown out highlights. The solution is to shoot a five stop (EV) bracket and use layer masking in Photoshop to capture the highlight detail from the dark shots and the shadow detail from the light shots
Also see tutorial on Basic Image Processing Also see the tutorial on Bracketing and use of Layers and Layer Masks
©2003 Larry N. Bolch