How does something go out of focus?

Out-of-focus photo

The depth of field is relatively deep in a digital camera, since the focal distance from the lens to the CCD is extremely short. This means that normally, subjects within the effective autofocus range will almost always be in focus. Note, however, that the effective range is different for every model. When the center-weighted light reading AE (automatic exposure) is used for exposure control, the f-stop will automatically change through the steps f2/5.6/11, depending on the amount of light falling on the subject. This means that the focus will be shallow when the aperture opens for a dark image. This appears not so much as "out of focus," but as a loss of image quality.

A better photo under identical conditions

Compare:Understanding Printers/The edges of the principal subjects are unclear.