Most of the time, autofocus will do a really good job of getting your subject in focus. Even though I never put my camera in green box mode anymore, I usually leave autofocus on. I keep an eye on it, though, just to make sure that it’s doing what I want. This is especially true in tricky situations where autofocus might not be smart enough to get me the results I’m looking for.
Here are some situations where just blindly autofocusing might not be your best bet.
You’re shooting through a fence or other foreground object. If there’s something in the foreground with strong lines, such as a fence, your camera is likely to focus on the foreground object rather than your chosen subject. In this example I’ve manually focused on the container and blurred the fence, but if I’d left it up to the camera I probably would have had a sharp fence and a fuzzy shipping container.
Macrophotography. Macrophotography is challenging because you have razor-thin depth of field, so you have to pay careful attention to what parts of the image you want to have in focus. If you let the camera pick out what it wants to focus on, it will probably guess wrong. Serious macrophotographers generally use manual focus for everything.
Everything in your image is soft. When everything in your image is cloaked in fog, your camera might not be able to find anything to focus on. This beautiful image from award-winning nature photographer Joe Decker is titled Tree Ballet and Pogonip, and is an excellent illustration of the foggy, soft image. This image is from his series Mono in Winter. See more of Joe’s work at Rock Slide Photography.
Your subject is extremely off-center. Again in Joe’s image, even if the trees hadn’t been shrouded in fog the camera might not be able to focus on them. Autofocus systems tend to be heavily weighted toward the center of the image. If you stick your subject way off in a corner then your camera is probably going to lose this particular game of hide-and-seek.
You’re shooting portraits. Portraits are often shot with shallow depth of field, but the subject’s eyes should always be in focus. It’s OK for the subject’s nose to be a little bit soft, and the ears can be downright blurry, but the eyes had better be crisp. If you let autofocus have its way then it might decide that it really wants the ears or the shirt or the background to be sharp, and leave the eyes shrouded in some of Joe’s mist. In this particular example, the camera got overly enthusiastic about the sharpness of the bear’s nose.
You’re shooting in low light. Autofocus often has a horrible time finding focus if you’re shooting in very low light. If you can’t see very well, your camera probably can’t either.
Those are some of the key situations where just trusting your camera to get the focus right might not be the best idea. In the next lesson we’ll see what we can do about autofocus problems.