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Tuesday, April 26, 2011

Boost Your AF Performance


How to set up your camera and lens to get quick response when you’re shooting fast-moving subjects like wildlife




This Article Features Photo Zoom

If absolute fastest AF response is required, use only the center AF point. With lower-end DSLRs, activating all the AF points can slow AF performance noticeably as the camera’s processor has to deal with information from all the points. In this nine-point system, there are big gaps between points.
"Ballpark" The Focus Manually
3 Set the focus in the ballpark manually before activating AF. That way, the AF system will lock onto the subject much more quickly, with no hunting, and you’ll be ready to shoot much sooner. If the lens is focused near its minimum focusing distance and the subject is about 150 feet away, the camera may never achieve focus, and the image will be so out of focus, you may not even be able to find the subject when you look through the viewfinder. In keeping with the above, a vital feature of a bird-in-flight lens is the ability to prefocus manually while in AF mode. Canon USM lenses with focusing scales allow this, as do Nikon AF-S lenses, Pentax SDM lenses and Sony’s 70-400mm SSM zoom. If you have to switch to manual mode to prefocus manually, that will slow things down considerably.

Use Continuous AF
4 Use continuous AF for action shots obviously. Put focus in the ballpark manually, position the AF point on the subject, then partially depress the shutter button or the AF button to activate the AF system (see Tip #8: Program The AF Button). Pan the camera to track the subject, keeping the AF point on the desired spot, give the system a moment to do its thing, then fully depress the shutter button to make the shot. Don’t stop panning as you depress the shutter button; follow through.

Use The Focus-Range Limiter
5 Use the focus-range limiter on your long lens if it has one. Most animal action occurs far enough from you that you don’t need the lens to focus through its entire distance range, and having it do so slows down the process greatly.

Switch Off The Stabilizer
6 Image stabilization works best with stationary subjects or subjects moving at a steady rate in a steady direction. For erratically moving animals like flying birds, stabilization actually will fight you as you track the bird, and that doesn’t lead to sharp images. Stabilization also takes time—up to an additional second with some stabilizers. When you need the quickest response, you’ll get it with stabilization turned off.

Some feel that stabilization helps the AF system perform better, so it won’t hurt to try wildlife action shots both with and without stabilization, and see which works best for you with your gear.

Select The Focus-Change Delay
7 Some DSLRs let you choose how long the camera will wait to refocus if the focus distance changes abruptly. This is handy if a bird subject briefly flies behind a tree or if you let the AF point momentarily slip off the subject. If you set a longish delay, the camera won’t try to focus on the tree or background immediately. If you set minimal delay, the camera will immediately try to focus on the tree or background. I usually leave this set for “normal,” but you can experiment with your camera to see what works best for you. If the camera does lose focus on the subject, immediately let go of the shutter button, manually reset focus in the ballpark, then press the shutter button halfway down to activate the AF system again.

A side note: AF systems work best with subjects against a plain background. I have yet to use a DSLR that was good at tracking a subject against a busy background. So try to photograph action subjects against plain sky or a large water surface rather than against busy foliage.

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