What is an F-Stop?

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  • Written By: A Kaminsky
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  • Last Modified Date: 03 October 2019
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Every profession has its own lingo, and photography is no exception. One is likely to hear a photographer say something like, "Open up that aperture" or "Bring it down one stop." These phrases refer to the same thing: the f-stop. The f-stop is a term for a measurable expression of how much light is entering a camera lens.

All cameras have a lens which helps record the image. Light must come in through the lens in an exact amount for the resulting photograph to be properly exposed, however. That is, the photo should neither be too light nor too dark. The f-stop on a camera helps regulate the exposure.

The "F" in f-stop stands for "focal length." The focal length divided by the pupil diameter, or the amount of light entering the lens determines the f-stop number. It is often expressed as something like "f/16" or "f/3." The number denotes the width of the opening in the aperture, which is an opening behind the camera lens. The aperture works rather like the pupil of the eye, and swirls narrower or wider, depending on the desired amount of light. Odd as it sounds, the larger the f-stop number, the smaller the opening in the aperture. F/16 is a smaller opening than f/3.


In a technique called reciprocity, a photographer can achieve similar results using seemingly opposite methods. For instance, he or she can use a wide f/stop and fast shutter speed, or a narrow f-stop and a slow shutter speed. Both will result in a properly exposed photo. The f-stop also provides what is called "depth of field," however.

Depth of field is how much background is visible behind the main object in the photo. For instance, if a photographer wants to show a single flower in a field, he will use a wide f-stop and a faster shutter speed. This will blur the background, bringing the flower into sharp focus. If, however, the photographer is taking a shot of friends in front of a mountain vista, she will use a narrower f-stop and a slower shutter speed. This enables the photographer to capture her friends in the foreground, as well as the mountains behind them, with all in sharp focus.

The f-stop works in conjunction with shutter speed in measuring how much light enters the lens. Shutter speed measures the exposure time for the photograph. Shutter speed is expressed in hundredths of a second. A 200 shutter speed, for example, is two-hundredths of a second. This is why a narrow f-stop and slower speed, or a wide f-stop and faster shutter speed allow approximately the same amount of light into the lens — only the depth of field differs.

Since many photographers take their photos on sunny days, the old rule of "sunny 16" is worth remembering. If the camera has manual settings, set it for f/16, with a shutter speed corresponding to the film speed. In these days of digital cameras, set the shutter speed and ISO for the same speed. Bingo: perfectly exposed photos every time.

TThere are many more permutations of f-stop use and technique, depending on the size of the lens, ambient lighting, what is being photographed, and other factors. A basic understanding of the f-stop will aid a beginning photographer in getting more out of his camera, however.


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Post 5

Using an aperture-priority mode that is offered on most all digital single lens reflex cameras can help beginning photographers achieve the proper depth of field and still have some automatic control over the exposure.

For instance, if you wanted a shallow depth of field effect but don't know what the proper corresponding shutter speed should be, the camera can automatically calculate the value based on the metering coming through the lens.

Shooting "wide open" is my personal preference and can aid in blurring out undesirable background artifacts that would be distracting if they were sharp in the image.

Post 4

Liked the article until I got to ISO. What is an ISO?

Moderator's reply: The ISO denotes how sensitive the image sensor in the camera is to the amount of light present. Thanks for visiting wiseGEEK.

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