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.
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Source: How-To Geek
