Understanding Histograms

It is tempting to judge the exposure of a photograph you have just taken by how light or dark it looks on the back of your camera. However the same shot will look totally different when viewed in a dimly lit room than when viewed in full sun. The image has not changed so clearly it is a very unreliable way to assess an image

The Histogram is a graphical representation of the exposure.
Running from black at the left hand side to white at the extreme right. Naturally enough the midtones are in the middle!
The Histogram above is typical of a subject with a range of tones that includes black, shadow areas, midtones, highlights and white. When you know how to interperet a Histogram you have a much better method to check exposure.

Exposure refers to the amount of light that strikes a camera's sensor to form the photographic image.

The human eye can cope with an enormous brightness range. For example on a bright day we can see in to the deep shadows cast by a tree at the same time we can see detail in the lightest part of a white cloud directly lit by the sun.

The camera's sensor can only capture a more limited brightness range in a single exposure. If you want to record the subtle detail in the lightest part of the cloud then detail will be lost in the shadow ares of the image. Similarly if you set exposure to reveal full detail of subjects in shade then you will end up with "washed out" highlight detail where light tones become featureless white.

A "correct" exposure can be regarded as one where the most important areas of the image look "right".

You can get an idea of how a sensor sees the world by squinting. With your eyes very nearly shut you will notice that highlights are virtually unchanged but shadow details disappear.

The image of the white church (right) used the full Dynamic Range of the sensor from the brightly lit white walls to the shadows inside the arched entrance.

Note: If the sky is overcast there is a lot less contrast and it becomes easier to fit the range of subject tones into the "dynamic range" of the sensor.