Camera Basics: Aperture

by on October 5th, 2015
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There are three factors involved in the exposure that captures an image: aperture, shutter speed, and ISO equivalent. Of these, aperture has the biggest effect on most pictures. Understanding aperture, what it does and how to choose the best setting for your picture is critical to getting the results you want.

Simply put, the aperture is the opening in your lens through which light enters the camera and falls on the film or sensor. A smaller aperture lets through less light, and a larger aperture lets through more light. Camera lenses incorporate a device called an iris which opens and closes to provide various aperture settings.

Camera manufacturers use the f/number to describe the amount of light the lens aperture is passing into the camera. The f/number is defined as the ratio of the lens’ focal length to the aperture diameter. For example, a lens with a focal length of 50mm that has an aperture diameter of 25mm would be set at f/2. Change the aperture diameter to 12.5mm and you have f/4. Smaller f/numbers let in more light, and larger f/numbers let in less light.

In photography, an f-stop is a change in the aperture setting that reduces or increases the light entering the camera by 50%. This means changing the area of the opening by 50%. So, if your camera is set at f/2, how do you know which f/number is one f-stop less? Most photographers just memorize this progression of f/numbers: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. Each number in the sequence is one f-stop from the next, so f/2.8 passes 50% less light than f/2 and 50% more light than f/4. Modern cameras often allow fractional aperture settings between the traditional f/numbers listed above.

Depth of Field
In addition to controlling the amount of light coming into the camera, aperture also determines the depth of field in your photo. Depth of field is the amount of linear distance which appears to be sharply focused. The smaller your aperture, or the larger your f/Number, the larger your depth of field will be. For example, let’s say that you are using a DSLR with a 50mm lens and have focused on an object that is 10 feet from your camera. With the aperture set to f/4, your depth of field begins at 9.12 feet and ends at 11.1 feet. If you change to f/16, your depth of field begins at 7.23 feet and ends at 16.2 feet. The depth of field provided by any given aperture is dependent on the focal length of the lens, the focusing distance and the size of your film or sensor, so I suggest the use of a depth of field calculator. There are many available on the internet and as apps for smart phones.

How do I Choose an Aperture?
The aperture you choose for a particular photo is one of your primary creative controls. You’ll have to experiment a bit to get a feel for what a particular aperture does in a given situation. Still, there are some guidelines that are helpful. If you want to emphasize the subject of your picture and minimize distracting background elements, a wide aperture (small f/number) will leave the background out of focus while keeping the subject sharp. When shooting scenes or subjects that have a lot of depth, like a landscape with a prominent foreground element, a small aperture (large f/number) will allow you to keep everything in the picture sharply focused. A good way to learn quickly about the effect of different apertures is to take the same picture with different settings, then compare the results. In no time at all you’ll be choosing your aperture settings with confidence.

I hope you’ve found this article useful! Best of luck in your photography, and look for more articles in my Camera Basics series.

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