What Is the F Stop on a Camera?by Bridget Conn
The f-stop on a camera refers to its aperture setting. Aperture is one of the two exposure controls on a camera, the other being shutter speed. Aperture controls the diameter of the lens' diaphragm that lets light into the camera and onto the film plane or chip. Your aperture choice affects the visual outcome of your image and, in turn, what message you convey with your photograph.
F-Stop and Light
Your chosen f-stop number controls the width of the opening that creates a film or digital exposure. Photography is full of counterintuitive number relationships, and aperture is no exception. The lower the f-stop number is, the more light that enters the camera. Using a high f-stop number, which lets in little light, causes your camera in automatic settings to compensate by letting in more light with a longer shutter speed.
There are traditional f-stop numbers as they relate to film photography and its lenses. These are marked by "f/" and a number. The amount of f-stops any one lens has will vary, but they can include f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/32, f/45 and f/64. Because these numbers are based on ratios between aperture diameter and the focal length of film lenses, f-stop numbers are different for most digital cameras. Also, half stops or even third stops of exposure with aperture are possible with some film and most digital cameras.
Visual Effects of Aperture
The most obvious reason to change your aperture setting is to control the amount of focus present throughout the depth of the photograph, commonly called its "depth of field." If you are focusing on something close to the camera and want the background to be as blurry as possible, use a low-numbered aperture that lets in a lot of light to create a shallow depth of field. To bring as many planes of depth into focus as possible, choose a high-numbered aperture that lets in a little bit of light, creating a greater depth of field.
Aperture and Lenses
Any given f-stop number will not produce the same depth of field on a wide-angle lens as it will on a long lens. Wide-angle, or short, lenses inherently create very shallow depth of field. Therefore, while an f-stop of f/4 on a 28 mm lens may bring most planes of depth into focus, the same is not true when using that same aperture on an 80 mm lens. In this case, the plane of focus could, under particular circumstances, be only a foot wide or even less.
Aperture and Focusing
If you are in a fast-paced shooting situation that calls for as much sharpness as possible, it is best to use a short lens with a high f-stop number. Because this aperture setting has a wider depth of field, it is less likely any part of your image will fall out of focus. On the other hand, when using a long lens, you can't always preview just how much of your image will fall into focus. Because the depth of field is so shallow with long lenses, precise focusing is even more crucial.
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