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
Most DSLRs will autofocus with lenses and lens/teleconverter combinations that are ƒ/5.6 or faster. Some pro DSLRs can autofocus with lenses/combos as slow as ƒ/8, although AF speed is noticeably slower. AF performance is generally quicker in brighter light and with faster lenses, in part because the faster lenses are generally high-end pro models with better AF motors and focusing algorithms.
A number of DSLRs have some AF points that provide added precision when lenses of ƒ/2.8 or faster are used. This is due to the wider base they provide for the phase-detection AF system—ƒ/2.8 provides a wider light beam than ƒ/4 or ƒ/5.6. If your budget permits, you'll get the quickest and most accurate AF performance with a pro DSLR body and a fast pro AF lens, but today's midlevel cameras and lenses provide excellent AF performance, and even entry-level models can do birds in flight.
While today's manufacturing capabilities are amazingly good, it's still possible to get a lens that doesn't perform well with a particular camera body. This may be because one or the other is defective, but more likely, it's just that one is at one end of the manufacturing tolerance limit, while the other is at the other end.
A number of today's DSLRs have an AF fine-tuning feature that lets you compensate for such mismatches. If a particular lens consistently front- or rear-focuses on your camera body, you can use this feature to make the lens focus farther away or closer to the camera than it normally does. See your camera manual or the article "AF Fine-Tuning" in the July 2011 issue of OP, or online at www.outdoorphotographer.com/columns/solutions/af-fine-tuning.html for more information.
When To Focus Manually
AF is quick and convenient, and as mentioned, a boon to action shooters, but there are times when it's better to focus manually. If precise focus on a specific portion of the image or subject is essential, for example, when doing selective-focus shots of flowers, it's best to focus manually.
There are subjects and situations that can cause problems for AF—subjects with no contrast like a cloudless sky, subjects with extremely fine detail, dim light, very small subjects such as a distant bird or very bright subjects like glare on the water. If the AF system won't autofocus on a subject, you're better off making a fast switch to manual focus than trying to fight it.
Make sure the viewfinder diopter is set properly for your shooting eye. Point the camera at a clear sky or a plain wall, and adjust the built-in dioptric eyepiece correction until the AF target in the finder appears sharp.
AF For Video
Some DSLRs provide autofocus during video recording. This is handy, but has drawbacks—it's slow, can cause focus jumps and abrupt exposure changes, and the built-in microphone will record the sounds of the AF motor. Pro movie-makers generally focus manually during recording, and that's the best method with a DSLR, too. With a DSLR, either set focus before you begin recording and leave it there, or adjust it smoothly manually, if need be, during recording.
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