Exercise: Autofocus

Let’s play with our autofocus for a bit to get a feel for how it behaves. To start with, make sure your camera is set to use all of the autofocus points. If you’ve never changed this from the default, you should already have this set up. If you need to change this, consult your camera’s manual for specifics.

First exercise: Find an object that’s maybe three or four feet away from you, has sharp edges, and is reasonably well-lit. Turn the autofocus ring on your lens all the way to one extreme, so that you’re either focused on infinity or as close as your lens will go. Point the camera at the object, look through the viewfinder, and press the shutter button halfway. You should hear the lens whir, and you’ll see the object pop into focus. Pay attention to which of the autofocus points lights up. Now turn the viewfinder all the way to the other extreme and do it again.

Try this with faraway objects and nearby ones until you get bored with it.

Second exercise: Find a well-lit, solid-colored wall. Point the camera at it so that you have nothing else in the viewfinder besides the wall. Press the shutter button halfway and see if you can focus. You might be able to, but chances are pretty good that the camera will try for a while and then give up.

Third exercise: Find a piece of paper with some text on it, and put it somewhere with a decent amount of light. Alternately, you can use your computer screen. Get up close, so that the lens is only a couple of inches away from the print, then push the shutter button to try to focus. You probably won’t be able to. Pull back a couple of inches, and try again. Keep backing up a couple of inches at a time until the lens is able to focus.

So what did we learn?

In the first exercise, you watched the camera focus. Some lenses focus so quickly that you can barely see it happening. Other lenses are much slower and will take a significant fraction of a second to find focus. You got to see which autofocus points the camera used for different situations.

In the second exercise, the camera probably couldn’t find focus. If there were no surface details, the camera wouldn’t be able to find any edges, and therefore it wouldn’t be able to focus. The period of time when the lens was searching for something to focus on is called focus hunting.

Even one small feature like a nail hole on the wall would probably be enough for the camera to focus on.

In the third exercise, you found the minimum focusing distance of your lens. Every lens has a minimum distance at which it can focus, and if you try to focus on something closer it won’t work. Some lenses, especially macro lenses, can focus very very close. Others may require you to be a couple of feet away from your subject. The minimum focusing distance is probably written on the barrel of the lens.

I did this exercise with the Canon 50mm f/1.8 “nifty fifty” lens, which has a minimum focusing distance of 1.5 feet. My most extreme lens, the Canon MP-E 65mm macro lens, has a minimum focusing distance of around one inch. Sometimes a lens will have more than one minimum distance, and a switch that will let you control it– this is called a focus limiter. The Canon 70-200mm f/4L IS lens, my usual telephoto lens, can be set to either 1.2 meters or 3 meters. A focus limiter lets you help the camera focus more accurately by limiting the range at which the camera and lens try to focus. You don’t really need a focus limiter, but in some situations it can be a useful tool.

Now, go out and play! If you have more than one lens, try them all. Try focusing in a very dark room. Try focusing on something that’s not in the center of the image. Make up your own experiments and get a feel for how your equipment works. In our next lesson we’ll use this knowledge to help you focus in challenging situations.

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