What’s an F-Stop?

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The f-stop is a term used in photography to measure the amount of light entering a camera lens. It is expressed as a number such as f/16 or f/3, indicating the width of the opening in the aperture. The f-stop works with shutter speed to adjust exposure and depth of field. A basic understanding of the f-stop is helpful for beginner photographers.

Every profession has its own lingo and photography is no exception. You’ll likely hear a photographer say something like, “Open that aperture up” or “Turn it down a 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 that helps record the image. However, an exact amount of light must enter through the lens for the resulting photograph to be exposed correctly. That is, the photo should be neither too light nor too dark. The f-stop on a camera helps adjust exposure.

The “F” in f-stop stands for “focal length”. The focal length divided by the diameter of the pupil or the amount of light entering the lens determines the number of f-stops. It is often expressed as something like “f/16” or “f/3”. The number indicates the width of the opening in the aperture, which is an opening behind the camera lens. The aperture works in a similar way to the eye’s pupil, and swirls narrower or wider, depending on how much light is desired. As strange as it sounds, the higher the number of f-stops, the smaller the aperture in the aperture. F/16 is an aperture smaller than f/3.

In a technique called reciprocity, a photographer can achieve similar results using seemingly opposite methods. For example, he may use a large f/stop aperture and fast shutter speed, or a small f/stop aperture and slow shutter speed. Both will result in a properly exposed photo. However, the f-stop also provides what’s called a “depth of field.”

Depth of field is the amount of background visible behind the main object in the photo. For example, if a photographer wants to show a single flower in a field, he or she will use a wider f-stop and a faster shutter speed. This blurs the background, bringing the flower into focus. If, however, the photographer is taking a photo of friends in front of a mountain view, he will use a narrower f-stop and slower shutter speed. This allows the photographer to capture his friends in the foreground, as well as the mountains behind them, with everything in focus.

The f-stop works in conjunction with shutter speed to measure the amount of light entering the lens. Shutter speed measures the exposure time of the photograph. Shutter speed is expressed in hundredths of a second. A shutter speed of 200, 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 roughly the same amount of light into the lens—only depth of field differs.

Since many photographers take their photos on sunny days, it is worth remembering the old “sunny 16” rule. If your camera has manual settings, set it to f/16, with a shutter speed that matches the film speed. In these days of digital cameras, set your shutter speed and ISO to be the same speed. Bingo: Always perfectly exposed photos.
There are many other permutations of f-stop use and technique, depending on lens size, ambient lighting, what is being photographed, and other factors. However, a basic understanding of the f-stop will help a beginning photographer get more out of their camera.

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