Autofocus is kind of like a microwave oven. Just a few short decades ago, it didn't exist. And now we have no idea how we could live without it.
Today, the idea of shooting all of our images with manual focus seems a bit crazy. But here's the thing, and you may find it hard to believe - there are some shooting situations where you need to shut off your autofocus. There are some shooting situations where your autofocus is going to hinder you instead of help you.
The simple reason why is this: your autofocus system just isn't that smart. It uses certain visual cues to know where to focus, but it does all of that based on contrast and light. It isn't able to intelligently look at a scene and understand what the subject is supposed to be, so it has to guess. In most cases, it gets it right. But there are certain situations where it's almost always going to get it wrong. Here are the five most common:
Low Contrast Scenes
Have you ever tried to take a picture of something all-white, or mostly white? Let's say you've got some white eggs and you want to put them in a white bowl and photograph them on a white background. That's going to make a pretty cool and interesting photo, but when you point your camera at all those glorious white objects, what happens? Probably this: your camera focuses in, and then out. And then in, and then out. This is called "hunting", and quite frankly it is one of the most irritating things that my camera ever does.
The reason why this happens is because of the lack of contrast in that white-on-white scene. Your lens needs contrast in order to lock focus, otherwise it has no idea where those edges are. To get around this problem, you can try placing the most defined edge in the scene in the center of the frame (or wherever your focus point is), press halfway down to lock focus, then recompose and shoot—but you may find that even that strategy doesn't work. If so, you'll need to turn off your autofocus and focus manually.
Egg by Flickr user Gabriel 'Briel' Rocha
Low light situations present pretty much the same problem as those low-contrast scenes do. Your camera may be able to get a great exposure in a low-light situation, provided you use a tripod and a slow enough shutter speed, but it can't see well enough in the dark to know where to focus. Some cameras try to get around this problem with an "AF assist lamp," which is a small light located above or next to your lens. This light will temporarily illuminate your subject long enough for your camera to lock focus, (with the unfortunate side benefit of annoying human and animal subjects, who generally don't like to be blinded by small lights. Some cameras use infrared to get around the too-bright problem). AF assist also has limited applications, because you need to be within 10 feet of your subject in order for it to work, so that means it's out for shooting after-dark landscapes or larger scenes. When all else fails, you'll need to switch to manual focus and use your own personal night vision to ensure that the scene is sharp.
Snow-blasted trees by Flickr user tricky (rick harrison)
Glass, Wire and other Near Obstructions
Here's another scenario you're probably familiar with—you're at the zoo, and you'd really, really like to get a nice shot of that tiger. He's out there basking in the sun, and your zoom lens will let you get just close enough to fill the frame. It's the perfect opportunity for a stunning shot.
Unfortunately (actually fortunately, because no one likes to get eaten by a tiger), the zoo has thoughtfully installed a piece of thick glass between you and Shere Khan. Your lens won't stop doing that hunting thing that is so annoying. This happens because your lens sees reflections in the glass, as well as smudges, fingerprints and that soda that some kid poured over the front of the enclosure. Your lens thinks it should be focusing on whatever is closest to it, which is that piece of glass with the sticky spilled soda.
There are a couple of things you can try to try coaxing your autofocus into working—besides moving to a non-soda-stained part of the enclosure, the first is to change your angle so that the reflections are minimal or completely absent. The second is to put your lens flat against the glass, which will also block out reflections (a rubber lens hood can help with this). But you may find it a lot less frustrating if you just switch off your autofocus and manually focus on the tiger instead.
This also happens anytime you're trying to shoot through something—say the tiger enclosure is behind wire instead of glass—then your camera is going to try to focus on the wire instead of on the tiger. Natural frames such as the leaves of a tree or anything you're reasonably close to will present a similar problem—be prepared to switch over to manual focus if your lens starts hunting.
Amur Leopard by Flickr user digitalART2
Here's the thing about macro—when you get really close to your subject, your autofocus gets confused. That's because at very close range, depth of field is really, really shallow—in fact the distance between that tack-sharp focus point and the point where everything starts to fall out of focus could be a matter of millimeters. Now theoretically, you could still get autofocus to work if your subject and camera were both very, very still and you use single-point AF to precisely locate your focus point. But most of the time, your autofocus is going to hunt and refocus just when you had the scene exactly the way you wanted it to, because your autofocus really doesn't know where you want to place that focus point.
When you're shooting macro, it's generally a good idea to switch to manual focus, and to place your camera on a tripod. That's because even when you're focusing manually, most people don't have nearly the sort of steady hands you need to keep that focus point sharp when hand-holding your camera. Even a little bit of movement can completely change the focus in the scene, so it's best to stabilize your camera and use a remote release, even when you're using manual focus.
Open for business by Flickr user Darwin Bell
Fast, Close Action
This may seem a little counter intuitive, because you and I both know how difficult it is to manually focus on a moving subject. But as good as that focus-tracking system is in your DSLR, it's not quite good enough to track very, very fast action—especially when it's moving towards or away from you, or it's very close to you (either physically close, or brought closer via a zoom lens). So while you'll have lots of success with your autofocus and focus tracking system shooting from the grandstands at a high school football game, or shooting those darling little kids running around at a birthday party, if you try to shoot something like a motocross rider, or you try to fill the frame with a very fast-moving RC toy, your autofocus won't be able to keep up. The problem is compounded by subjects who are moving towards you or away from you—the changes in distance are just too pronounced for your relatively slow-moving autofocus.
For these situations, you're going to have the best luck if you switch to manual focus, and then prefocus on a part of the scene where you know there will eventually be some action. Then wait patiently for the action to arrive in that spot, and take the photo.
Autofocus is a great tool, and most of the time it's going to serve you very well. So don't think for a second that this article is meant to endorse only the use of manual focus, because manual focus will most likely slow you down if you switch to it and stay there. But do take note of the situations above, and when you're in one ask yourself if it wouldn't be a good plan to use manual focus for a while. It's almost always going to give you better pictures in those difficult situations, and perhaps more importantly, it will save you from having to tolerate all that super-annoying hunting that your lens will do when left to its own devices.
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