How Aperture Controls Depth of Field

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Aperture is responsible for a number of things, including exposure, sharpness and depth of field. Speaking of the latter, let's have a look at how aperture actually influences depth of field.

First of all, let's assume you want to photograph a friend outdoors. You want to take a regular portrait, from the shoulders up. Regarding focal length, it's best to shot with at least a 50mm lens, if not longer.

The best way to understand how this works is to experiment with different settings. A smaller aperture, like f5.6 or f8 will always reveal more; therefore it's not the best option for separating the subject from background with a shallow depth of field. Things become sharper as less light enters the lens.

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If you take the exact same shot at an aperture setting of f/2.8 or lower, you will notice a big difference. The background is going to be a lot less in focus and the eyes will focus on the subject, just as intended. Naturally, the wider the aperture, the more blur you'll have behind the subject. A lens with a maximum aperture of f/1.2 or f/ 1.4 will do wonders, but it will also be trickier to use. It's a lot more difficult to get even the subject in focus at this aperture setting, and even the slightest camera move can shift focus. Generally when shooting portraits, it's best to put the focus points on the eyes, but with an f/1.2 lens, if you get too close to the subject, the depth of field will be so shallow that everything behind the eyes will be out of focus.

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Here's Mark Wallace with a detailed explanation about the relationship between aperture and depth of field, in a video made for Adorama TV.

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