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Tuesday, November 18, 2008

10 Tips For Better Autofocus


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10 tips
7 Shenandoah National Park, Virginia
9 Working With Teleconverters And AF
Modern teleconverters that are designed for specific focal lengths are very good. They might not match a single-focal-length lens in absolute sharpness, but on a cost-benefit basis, a teleconverter’s value can be huge. The problem is that they significantly reduce light to the focal plane of the camera and that results in less light to AF sensors. Sometimes this means that the autofocus doesn’t work at all with a teleconverter and a lens. In this case, especially, you may be required to use a fast lens with a teleconverter to coax your AF system to work.

10 Help Out Your AF System
It sometimes can take a fraction of a second to find an object and lock focus. Give your camera and lens a head start by starting the autofocusing early. If you wait until the last minute, you may find that you can’t get the photograph you want because focusing will be trailing the action. This is especially important for moving subjects, such as flying birds. Start by pressing the shutter button lightly, which engages your autofocus. If your camera has a specific AF button on the back of the camera, use it to start your autofocus (the camera is only focusing and not setting exposure or setting off the shutter).

Lenses and systems that allow manual focusing at the same time that your autofocus is on can help, too. Do a little prefocusing manually before you need your autofocus to work. That way, the AF system doesn’t have to search for something to focus on as you have already given it that information.

Cross-Type Sensors
A number of D-SLR (and 35mm SLR) manufacturers point out that some (and with a few models, all) of the AF sensors in their cameras are cross-types. Early phase-detection AF cameras used line-type sensors, which could only read focus with subject lines perpendicular to them: A horizontal sensor could read vertical lines but not horizontal ones; a vertical sensor could read horizontal lines but not vertical ones. A cross-type sensor, as you might suspect, can read both horizontal and vertical lines and thus is able to focus on a much greater number of subjects than a horizontal or vertical line sensor. Some cameras even employ diagonally oriented cross-sensors, so that they can autofocus on diagonal lines in a scene or subject.

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