A Viewfinder Darkly

Photography tips and tutorials

Beat Camera Autofocus Problems

April 3 2007

by Philip Northeast

There are times when the automatic focusing mechanism on expensive Digital Single Lens Reflex Cameras (DSLR) will not work. Despite DSLRs 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.

autofocus may struggle in low light situations

In brightly lit situations where there is a simple subject whose colour 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 colour to the background colour 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 cannot decide on one to use. This same principle applies when there are a lot of objects in the focus range.

Autofocus in Sports Photography

Normally in autofocus mode a digital camera will not allow the shutter to operate until it decides it is in focus. So when that magic moment arrives and the play of the year unfolds in your viewfinder, the digital camera decides it is not quite happy with the focus and delays the shutter until after the critical moment. This possible delay is one of the reasons that sports photographers pre-focus on a spot where they anticipate the action will occur.

It is common for sports photographers to use long focal length lenses to get close up shots of the action and to use wide aperture settings to allow fast shutter speeds resulting in shallow depth of field. This can make accurate focusing critical and photographers devote considerable effort to making sure the focus point is where they want it. Think about your sport, this could be around the goal, start or finish lines or corners and turning buoys.

When set to continuous autofocus mode, the digital camera will keep a moving object in focus, even though it may be moving closer to or further away from the camera. It is possible to refocus while tracking a moving object but this technique requires skill and lots of practice.

Landscape Photographers – Hyperfocal Focusing

Hyperfocal focusing is an advanced technique in landscape photography for getting as much of the image in focus as possible. The point of focus is set using the distance scales on the focusing ring rather than selecting a particular object. Some street photographers use the hyperfocal technique. One of the advantages of this technique is that it allows opportunistic shooting without having to wait for the autofocus system.

With the almost continuous rush of new cameras, the performance in these weak spots is constantly improving. For the prospective new camera buyer this is an area of meaningful performance comparison in an era when even the worst camera is still very good.

originally published on suite101.com


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