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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

This Article Features Photo Zoom

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 all AF points are activated in single-shot AF mode, the camera will focus on the closest object to fall under one of the AF points. In continuous AF mode, you generally have to establish focus on the moving subject using the selected AF point, then the camera will automatically switch to the other points if the subject moves off the selected point.
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.

AF Modes
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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