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What is Camera Aperture in Photography and What Does it Do?

A beginning photographer has a lot of new terms to work into his or her vocabulary. Some of them can be difficult to grasp, especially when they're associated with other, more technical terms like “f-stop”. One of those words is “aperture” and we're about to explore it thoroughly. When you're done reading this post, you should know exactly what it means, why it's important and how to use it effectively.

An aperture is an opening.

Like most photographic terminology, the basic definition of this word is simple. If you look it up in the Oxford English Dictionary, you'll find the first definition is “An opening, hole, or gap.” We can even learn from that same page that, to a photographer, it means “A space through which light passes in an optical or photographic instrument, especially the variable opening by which light enters a camera.

Where is it?

In modern SLR, DSLR and mirrorless cameras, the aperture is located between the elements of the lens. It's created by a mechanism called a diaphragm that controls the size of the opening, much like the iris of your eye. That's what you're controlling when you make an aperture adjustment. Most lens diaphragms consist of several overlapping “leaves” that rotate to adjust the size of the opening.

What does it do?

Coincidentally, the diaphragm also performs the same function as the iris of the human eye – it limits the amount of light that's allowed to pass to the film or sensor in the camera while the shutter is open. 

The size of the aperture, combined with the length of time the shutter is open, determines the effective amount of light that's allowed to strike the image recording medium. Aperture size and shutter speed are two of the three factors in the “Exposure Triangle”,  a formula that can be used to the exposure of an image.

There's another important function of the adjustable aperture, and we'll cover that in a moment. First, let's make sure you understand aperture size.

F-Stops and F-Numbers

You've probably heard a photographer say something like, “decrease the exposure a stop” or “push it a half-stop.” A “stop” in terms of exposure is the adjustment needed to double or halve the total exposure value (Ev). Modern digital cameras can be adjusted in much smaller increments, but we still use the f-stop as the basis for exposure settings.

Aperture sizes are expressed in f-numbers. Each number represents the reciprocal of its face value multiplied by the focal length of the lens. In other words, an aperture setting of 4 (f/4) on a 50mm lens represents:

¼ x 50 = 12.5mm (the actual size of the aperture)

An aperture setting of f/16  on the same lens represents:

1/16 x 50 = 3.215mm

So, the higher the f-number, the narrower the aperture is and vice-versa.

Calculating actual aperture sizes can get much more complicated, since modern cameras allow you to adjust the aperture in ½ stop or 1/3 stop increments.

Here's a typical full-stop aperture scale:

1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22, 32

Here's a typical 1/3 stop scale:

1.0, 1.1, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.5, 2.8, 3.2, 3.5, 4.0, 4.5. 5.0, 5.6, 6.3, 7.1, 8.0, 9.0, 10, 11, 13, 14, 16, 18, 20, 22, 25, 29, 32

Crazy, huh? Fortunately, you don’t have to know the actual aperture size to use this element of exposure effectively. It is important to know the concept, to understand exposure as well as the next term we're going to explore:

Depth of Field

The aperture size also controls the depth of field of an image. That's the range of acceptable sharpness in a photo, measured in distances from the focal plane of the camera. The range changes with the aperture setting, and shifts with your focus setting. Think of it as an area of sharpness that moves back and forth between your camera and infinity as you focus.

Depth of field (DoF) is inversely proportional to aperture size. That means that you can use a wide aperture (f/2.4, for instance) to isolate a sharp subject against a blurred background. On the other side of the scale, you can use narrow apertures like f/16 or f/22 to maximize the sharpness from front to back in a landscape photo.

Calculating depth of field is a complex operation based on several factors, starting with the focal length of the lens and the effective aperture size. It also depends on the size of your camera's sensor.

Fortunately, there are a few tools available. You can use an online calculator like this one. You may even have a valuable aid right on your lens. Many lenses have a DoF scale that looks something like this:

In this photo, the DoF scale is the numbers 4, 8, 11 and 16 located on either side of the focusing mark. Notice how the numbers on this scale align with the distance numbers of the focusing ring (the upper ring). For instance, the 4's are approximately aligned with 3.7ft (0.8m) and 9ft (2.2m). That's where the area of maximum sharpness will begin and end at this focus setting if you select an aperture of f/4. Selecting f/8 will give you a range of about 2.5ft (0.75m) to infinity, and so on.

In the photo above, the focus has been shifted to slightly over 3ft. Note that f/4 now covers about 2.6ft (0.8m) to about 4.6ft (1.4m) and so on.

There's no substitute for getting the DoF right in an image. Post-processing tools like Luminar can help somewhat, but getting it right in-camera is much more effective.

That's it for the basics!

There are other things associated with aperture, such as diffraction, bokeh and hyperfocal distance, but  those are separate lessons. By now you should have a good working knowledge of aperture and how it affects your photos. Go practice!

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