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There are times when the automatic focusing mechanism on expensive Digital Single Lens Reflex Cameras (DSLR) will not work. Despite digital SLRs being at the leading edge of technology, there are situations when it just gets too hard for the computer in the camera and it needs a photographer to take over. On other occasions focusing is a critical part of the creative process and the autofocus system would just get in the way. This might not be highlighted in all the glossy brochures, but there is probably a section in your digital camera's Operating Manual dealing with likely conditions that confuse the autofocus system.

Confusing the Autofocus System

Autofocus systems on digital cameras work by sensing the difference in contrast between edges of objects seen through the lens. When there is a sharp change in contrast at the edge of an object it is probably in focus. If the edge of the object is fuzzy and not clearly distinguishable from the surrounding background then it is not in focus and the digital cameras autofocus system will keep searching until it finds a nice clear sharp edge.

In brightly lit situations where there is a simple subject whose color makes it stand out from the background, autofocus is a breeze. However, if the light is a bit dim and the subject is similar in color to the background color and has fuzzy edges, then forget it. If your subject has patterns of lines within the autofocus target area there may be too many edges and the system can not decide on one to use. This same principle applies when there are a lot of objects in the focus range.