Autofocus is one of those modern marvels that we completely take for granted, like Bluetooth headsets and hybrid fruits that are half apple and half pear. It’s almost magic, though, when you think about it: Even the cheapest camera is pretty good at precisely focusing on your subject most of the time.
Unfortunately, though, there are a lot of situations in which your camera isn’t especially good at autofocusing. Either your camera gets it wrong or it just takes too long. Let’s look at some situations in which you should consider turning your autofocus control off and doing it the old fashioned way.
Before we get to that, though, it might be a good idea to review how your camera’s autofocus control works. When you press the shutter release, the camera quickly focuses on whatever is in the middle of the viewfinder. This generally only takes a fraction of a second, so the lag between when you press the button and when the photo is snapped is typically almost imperceptible.
But what if you don’t want to focus on what’s dead center in the viewfinder? This is where it comes in handy to pull out your camera’s user guide. Some cameras use built-in logic to sniff out the real subject in the frame and focus on it, regardless of whether it’s dead center or not. You’ll want to read the manual so you know how to control that option. Also, many cameras come with focusing zones–by moving little boxes around in the viewfinder, you can specify whether the camera focuses in the center, or the left, right, top, or bottom of the frame.
There’s an easier way, though, that works with pretty much any camera. Apply just a little pressure to the shutter release–press it halfway down–to activate the camera’s autofocus. It’ll lock onto the subject in the middle of the viewfinder. Then, maintaining light pressure on the shutter button, you can recompose the shot to your heart’s content. When you finally mash all the way down, you’ll get a shot with the focus set the way you like. Here, for example, is a classic case of recomposing the shot with the focus locked.
Beware of interference
Autofocus is pretty good, but it’s not perfect. There are a number of situations in which you might want to turn it off and rely on manual focus instead (check your camera’s user guide to see how to do that).
One of the most common such situations is when you’re shooting through a barrier, like a window, a screen, or a fence. Some autofocus systems get confused and have trouble focusing on the more distant subject. If you run into this problem, turn off autofocus and dial it in yourself.
You should be at infinity
What if you’re shooting a distant subject and time is of the essence? Say you're taking action photos of a sporting event, an animal in the distance, or fireworks. You’ll lose the shot if your camera takes too long hunting for focus, but here’s the rub–the focus should probably be set at infinity, anyway. Switch the camera to manual focus mode and spin the focus all the way to infinity and just leave it there; letting the camera try to focus just slows you down.
Your camera’s autofocus control might work like magic most of the time, but there are clearly situations in which it’s going to be off its game. Like low light, for example. Whenever the light is dim–and therefore the overall contrast is low–your camera will work harder to focus properly. If it struggles too much, it’ll take too long to take a photo, or the focus will simply be wrong. It’s probably easier to just focus the photo yourself, especially if the subject isn’t moving.
Shooting panoramas or HDR photos
Anytime you’re taking a sequence of photos and combining them digitally afterwards, it’s a good idea to ensure that the focus is identical in each shot. When you shoot a panoramic series, you typically want to set to infinity. You’ll be able to shoot faster if you set it to manual and dial in infinity yourself.
HDR (High Dynamic Range) photos are more critical, since your camera’s autofocus system might choose a different point to focus on in each shot. When you combine them on your PC to get a broader tonal range, the entire shot will look soft because you’re blending photos with different focus points.
When focus is critical
If you frequently use the “press the shutter halfway down to lock focus and then recompose the scene” trick, you might have noticed something: It doesn’t always work quite right. You’re not doing anything wrong; it’s just a simple case of geometry getting in the way. Imagine that you lock the focus and pivot your body to shoot something that appears to be the same distance away as the original subject. As you can see in the diagram, the distance from you to the foreground is now different, and you’re focusing just short of your subject. In some situations, that’s all it takes to create a slightly out of focus subject.