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Photography 101: Aperture and F-Stop

Photography 101: Aperture and F-Stop

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You’ve heard the terms “aperture,” “diaphragm,” and “f–stop,” but what exactly do they mean? They are all important elements of photography, and knowing how to use them will make your photographs great. In this guide, we’ll look at what these terms mean, how they work, and how you use them in your photography.

What the Terms Mean

Aperture” or “diaphragm” (the two are often used interchangeably) relates to the device that controls the amount of light that reaches the film or digital sensor in a camera. The f–stop is a term that’s used to indicate the speed of the lens, or the size of a camera’s aperture. The higher the f–stop number, the smaller the actual opening of the aperture is. The f–stop is an expression of the ratio between the focal length of the lens divided by the diameter of the opening.

How Aperture Is Determined

A lens’s maximum aperture is easy to understand with our 50 mm lens as an example. If you’ve ever looked carefully at the front of your lens, you’ve noticed that the maximum aperture is actually expressed as a ratio—something like 1:2 or 1:2.8. This is because the maximum aperture is actually a ratio between the focal length of the lens (50 mm, in our example) and the maximum diameter of the lens. A lens that is marked as “f/2” (this is the maximum f–stop setting) means that the diameter of the lens at its narrowest point is 25 mm across.

A cross-section of a 50 mm lens.

A cross-section of a 50 mm lens.

Many modern zoom lenses, especially the “kit” lenses that typically come with advanced amateur cameras, are variable aperture lenses (the 18 to 60 mm on my D40 has an aperture that changes from f/3.5 to f/5.6 as the lens is zoomed). This means that as the lens is zoomed out to long focal lengths, the ratio between the aperture and the focal length changes. In most situations, the camera will make the adjustment for you, so don’t worry about it for now.

How Aperture Controls Exposure

Changing the f–stop (or “stopping down,” as it’s called) causes a mechanical diaphragm inside the lens to close down and change this ratio. Now, here’s the thing you have to understand: as I said earlier, the aperture, or f–stop, is actually an expression of the ratio between the focal length of the lens divided by the diameter of the opening.

As the aperture or f–stop setting is changed, each whole stop lets in either half as much light, or twice as much light as the adjacent setting.

Examples of different lens apertures. A quick glance at the diagram should make things clear: as the aperture or f–stop setting is changed, each whole stop lets in either half as much light, or twice as much light as the adjacent setting. So a setting of f/8 allows in twice as much light as f/11, or half as much light as f/5.6.

This means that as the f–stop numbers seem to get bigger—f/8, f/11, f/16, and so on—the actual opening is getting smaller. So the bigger the f–number, the less light is actually entering the camera.

This point is often always slightly misunderstood. So, here’s another way to explain it:

On a 50 mm lens, an opening of f/2.0 means that the aperture has a diameter of 25 mm. At f/4.0, the aperture is only 12.5 mm in diameter; at f/5.6 the aperture is 8.9 mm in diameter. The f–numbers are getting bigger, but the physical diameter of the diaphragm is getting smaller.

Now that you know what aperture, f–stop, and diaphragm mean, it’s time to start shooting! For more photography information, be sure to check out our Quick Guides Getting the Right Color Balance in Your Digital Photography, and Photography 101: The Basic Conventions of Composition. Happy shooting!

From The Complete Idiot’s Guide to Photography Essentials by Mark Jenkinson