Technique: Making Autofocus Work

If you've used a digital SLR more than three or four times, chances are that you've had an experience where the image suddenly seems to go out of focus. Or it may be that you're scrambling around, trying to figure out how to focus in on a particular spot. Autofocus is great, but there are a few quirks and tips you typically need to know:

• AF Points A camera isn't a mind reader; you have to tell it where to focus. Most cameras use autofocus points (AF points, at least for the Canon owners). These are spots that you can see in the viewfinder. You should be able to choose which of the points to use, or whether they should try to average things out among them. If you have autofocus set for one point and that point isn't over the spot you want in focus, then chances are things will look fuzzy. Check to see which autofocus is active and set the point closest to the subject of your photo.


• Use Focus Lock If you are concerned that a picture might get away and don't want to start fussing with which AF point to use, then you can cheat. Move the camera so that the active AF point is over your subject, and then press the shutter button halfway. That generally locks in exposure and focus. Holding the button pressed part way, move the camera back to get the composition you want, and then press the button the rest of the way. You get the picture in focus.


• Look for AF Conditions There will be times that the camera keeps changing the focus but never settling in. That may be because either there's not enough contrast or that the image is too dark. Autofocus works by trying to maximize the contrast on edges in the image. If there is naturally low contrast in the subject, then the camera might not be able to figure out if it's in focus or not. Low light similarly makes the autofocus system crazed. In such a situation, there are two things I try. One is to find something else in the image that is about the same distance away from me. I focus on it, holding the shutter button half-way down, again, and then move back to the subject and finish pressing the shutter button. The other choice is to use manual focus, and there are occasions when that is the only choice that will work.


• Autofocus Switch If you've had to focus a lens automatically, or if you just wanted the practice, you might have forgotten to flip the autofocus switch back to automatic.