Tuesday, August 9, 2011
It's About Sharp
Take advantage of your DSLR’s sophisticated autofocus options to get perfectly sharp shots without missing any action
With most DSLRs, continuous AF mode is also predictive. There's a slight delay between the moment you fully depress the shutter button to make a shot and the moment the film or sensor is exposed. With a quick-moving subject, this can make a difference. The subject won't be at the same distance for the camera when the exposure occurs. To compensate for this delay, predictive AF calculates (from successive AF measurements) the subject's speed and direction, then adjusts focus to compensate for the change in subject distance between the time you press the button and the time the exposure is made. Obviously, predictive AF works best with subjects moving at a constant speed in a constant direction—if the subject changes speed or direction, the predicted position may not be correct.
Focus-Priority AF Vs. Release-Priority AF
Some DSLRs let you select between focus-priority and release-priority operation; others just offer focus priority. In focus priority, the camera locks the shutter button until it thinks the subject is in focus. This can prevent you from capturing decisive moments, as there's a delay between the moment focus is achieved and the moment the AF circuitry realizes it. With release-priority AF, you can trip the shutter at any time, whether or not the camera thinks the subject is in focus. Assuming you pay attention to what you see in the viewfinder, release-priority AF will get you those decisive moments you'd miss with focus-priority AF.
Most AF cameras offer multiple AF areas. You can activate all the AF areas and let the camera choose what its programming considers the best one or select one yourself. If you're shooting a close-up of an animal, you can choose the AF area that covers its near eye to assure that the camera focuses there when you compose the shot the way you want. For a lot of sports action or wildlife action, it's generally best to use the center AF point, as this assures that focus will be where you want it (if you activate all the AF points, the camera may not pick the one you would have chosen). If your subject is against a busy background, definitely use single-point AF, because with all points active, the camera is apt to focus on something besides the subject. For birds flying against a plain-sky background, you might try activating all the AF points—this will make it easier to keep a focus point on the bird, but can slow things down a bit unless you have a higher-end DSLR with a powerful processor. Some cameras let you activate groups of AF points rather than just one or all; this is handy when you want a larger AF area than a single point, but not the bulk of the image area.
In single-shot AF, the camera will lock focus once achieved until you take the shot or let go of the shutter button. You also can program the AF button on the back of the camera to lock AF—handy when you're in continuous AF mode and want to shoot a stationary subject.
If you're shooting stationary subjects at close range, like insects, it's better to activate the AF point that puts focus where you want it rather than using the center AF point, locking focus and recomposing. That's because at close range the distance from the sensor plane to the subject can change enough as you recompose to throw off focus. Most macro photographers focus manually, by setting the desired magnification, then moving the camera toward or away from the subject until the desired plane comes into focus.
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