How to Focus With Wide Aperture Lenses
Focusing is easy when you’re using an aperture of f/8 or narrower: most things in the scene will be pretty much in focus. When you start using wide apertures like f/2.8, f/1.8, or even f/1.2, however, you’ll start to miss focus a lot more. Here’s how to get the best results when focusing with wide aperture lenses.
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When we talk about focus what we’re talking about is sharpness. Say you’re shooting a portrait. Whether you’re using f/1.8 or f/16, the lens will still be focussed on the same point: the model. The difference is that the depth of field—or in focus terms, the range of acceptable sharpness—is much larger at f/16. Let’s look at this in action.
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Imagine you’re using an 85mm lens on a full frame camera with your subject 2.5 meters away. At f/1.8, the in-focus depth of field is just nine centimeters, four centimeters in front of the focal point and five behind it.
This means that if you focus on the subjects’ hand six centimeters of their face, their face is going to look blurry in the final image. You can see that in the shot below: the subject’s hands are in focus, but they’re far enough in front of his face that his eyes aren’t.
Imagine you switch to f/16. This time, you’ve got a range of acceptable focus of 82 centimeters, 35 centimeters in front of the focal point and 48 centimeters behind. This is a much much easier target to hit. You can focus on their outstretched arm and still probably get a good photo.
Aperture is only one of the factors that affect depth of field. The other major one is focal length. If you swapped to a 35mm lens and stayed the same distance from your subject, at f/1.8 you’d have a depth of field of 54 centimeters and at f/16, you’d have a ridiculous 72 meters. This is why what counts as a wide aperture gets narrower when it comes to telephoto lenses. On a 200mm lens f/5.6 is most certainly a wide aperture, but on a 17mm lens, it’s not. Follow the advice in this article whenever you think it will help.
Note that for these calculations, I’ve been using DOFMaster’s online calculator. It’s a great tool, and I’d suggest you spend a few minutes plugging in the gear you normally use to see what depth of field you get.
Right, with that covered, let’s dig in. With wide apertures, unless you’re using old gear designed for manual focus or locking your camera down on a tripod, you need to use autofocus. You won’t be able to focus on the fly manually. This means you need to get autofocus to work for you.
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Use A Single Autofocus Point
Every camera has multiple autofocus points. You can choose between all the different points, sub-groups of them, or a single autofocus point. I covered this in depth on the article on getting the most from autofocus.
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Generally speaking, a group of autofocus points hits the best balance in most situations. It gives you some control over where your camera is going to try focusing without being too restrictive. When you’re working with a wide aperture, however, you want to be restrictive. With a shallow enough depth of field, you can get a subject’s nose and eyebrows in sharp focus while their eyes are blurry.
To that end, you’ll get the best results when you use a single autofocus point—or possibly a very small group of points—placed directly over where you want your camera to focus. For good portraits, this means placing the active autofocus point directly on your subjects’ eye.
The only other autofocus option worth using with wide apertures is, if your camera supports it, eye detect autofocus. With it, your camera handles the job of placing the single autofocus point.
Use Continuous Autofocus
Similarly, your camera will have three different autofocus modes: Single, Hybrid, and Continuous.
Single autofocus works by searching for focus and then, once it’s found it, staying locked; great for landscapes but if you have a shallow depth of field and a moving subject you’re going to miss focus a lot.
Continuous autofocus, on the other hand, constantly tracks your subject; you might miss a few shots because your camera decides to focus on the background for a second, but it will be more reliable overall. It’s the one you should use.
Hybrid autofocus combines single and continuous autofocus. The problem is that when your depth of field is really shallow, hybrid autofocus may not adjust to small movements by your subject. For more, check out our article on the different autofocus modes.
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Shoot in Bursts
Even if you’re using a single autofocus point and continuous mode, you’re still going to miss a fair few shots. It’s just the reality of working with a really shallow depth of field. The good thing is you can pump your numbers by using burst mode.
Now, you don’t have to hold the shutter button down like you’re playing Call of Duty. It’s just that when you take a photo, instead of stopping after one, take three or four shots. Even if your subject moves the autofocus has time to catch up.
The other thing is, when you shoot bursts, you don’t have to worry about your subject staying still. You can encourage them to move, switch poses, and generally be active. You’ll get better results and capture more natural photos—as well as getting more shots in focus.