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|With many cameras, you can activate a group of AF points or the points in the center area instead of all the points. This is a good choice when you want to limit the AF area, but you want more than a single point.|
Nikon D300S Auto-Focus Mode Switch
Autofocus is one of the two most revolutionary features to appear in SLR cameras in the past quarter-century (the other being digital media replacing film). When it first appeared, many photographers saw a technology that had little value beyond encouraging a certain laziness in the field. Amid constant improvements, however, AF now is universally accepted as both fast and more accurate than manual focusing in most situations. Most of us don't think too much about the different AF modes and how they work, but you can get a higher percentage of keepers by knowing how AF works and knowing which mode is the best choice for a particular scene or subject.
Phase Detection Vs. Contrast AF
AF SLRs use an AF system called phase detection or phase matching. A portion of the light coming through the lens is transmitted through a small portion of the SLR mirror and split into two segments that are directed to a pair of AF line sensors. When the image is sharply focused, the two beams will strike the center portion of each AF sensor. When the lens is focused closer than the subject, the two beams will strike the inner portions of the AF sensors. When the lens is focused beyond the subject, the two beams will strike the outer portions of the AF sensors. Thus, a single reading tells the AF system whether or not the image is in focus, and if not, in which direction it's out of focus and by how much. This makes for quick operation and works well with moving subjects. Today's DSLRs have multiple AF sensors in their AF modules, but the principle is the same.
When Live View came to the DSLR, phase-detection AF presented a problem. The SLR mirror has to be in the down position for light to reach the AF module, while it has to be in the up position for light to reach the image sensor (necessary for Live View to operate). Thus, the live image is temporarily disrupted while the camera focuses.
For this reason, most DSLRs employ contrast-based AF for Live View, taking the focus reading right off the image sensor. The drawback with contrast-based AF is that it's relatively slow. The AF system reads the contrast, then adjusts focus slightly and reads it again. If the contrast increased, that means the original setting wasn't in focus, and the new one is moving in the right direction. The system then adjusts focus in the same direction, takes another contrast reading, and if the contrast increases, the image is closer to being in focus; if the contrast decreases, the previous focus setting was the in-focus one. If the second reading shows decreased contrast from the first reading, that means the first focus setting was closer to correct, so focus is adjusted in the other direction, and another contrast reading is taken. This makes contrast-based AF slower than phase-detection AF, and poorly suited to moving subjects. Contrast-based AF systems are much faster than they were just a few years ago, but they still aren't as good for action as phase-detection systems.
Types Of AF Sensors
When you look in the viewfinder of a DSLR and press the shutter button halfway (or view the AF-sensor layout diagram in the camera's instruction manual), you can see where the active AF sensors are located in the image frame.
Most of the sensors are line types, which are sensitive to only vertical or only horizontal lines. Most DSLRs also have at least one (the central one, possibly others) that's a cross type, sensitive to both horizontal and vertical lines. The cross-type sensors will focus on more subjects than the line-type sensors will. Some cameras even include a diagonal cross-type sensor, which further expands the range of focusable subjects.
DSLRs provide at least two AF modes: single shot and continuous. As you would expect, single-shot AF is for stationary subjects. The camera focuses on the subject, then locks focus there until you either take the shot or let go of the shutter button. Continuous AF is for moving subjects. The camera will keep focusing as long as you keep the shutter button depressed halfway. Some cameras also have a combination AF mode (Canon's AI Focus AF and Nikon's Auto-servo AF, for example), which uses single-shot AF unless the subject starts to move, at which point it switches to continuous AF. Note that the combo AF mode won't switch back to single-shot AF if the subject stops moving.
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You also can activate a single AF point to place focus just where you want it, such as this lion's eye.
With some cameras, continuous AF is stable enough that you can use it for still subjects as well as moving ones; with other cameras, a stationary subject may jump in and out of focus if it moves slightly (such as a bird preening) or you jiggle the camera a bit in continuous AF mode. See how your camera handles this. If it's stable, you can just use continuous AF all the time (handy when the camera makes it difficult to switch AF modes quickly). If continuous AF isn't stable on stationary subjects, switch to single-shot AF when shooting them.
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.
AF technology is evolving constantly. The new Sony Live-View DSLRs allow the normal phase-detection AF system to operate with no disruption as you shoot.
Note that the AF targets in the viewfinder aren't pinpoint precise; the AF sensor probably will "see" more than is included in each target. Keep this in mind when trying to focus on a subject with nearby distractions, such as a bird in a leafy tree. Some DSLRs allow you to change the size of the AF point, but not all.
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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This is a side view of a conventional AF system; the lens would be facing left, the image sensor at right. A transparent area of the mirror lets light through to the secondary mirror and AF mirror before coming to the AF sensor.
Effect Of Lens Speed
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.
|History Of AF
Minolta introduced the first widely successful AF SLR—the Maxxum 7000—in 1985. AF is certainly a boon for action shooters, and it speeds up all photography, although there are still times when it's best to focus manually. Historical note: The Pentax ME-F (1981) and Nikon F3AF (1983) predated the Maxxum 7000, but they were essentially existing manual-focus SLRs adapted to work with a limited number of lenses containing AF motors (one, in the case of the ME-F, two for the F3AF), and not commercially successful. The Maxxum 7000 started the AF SLR revolution.