An aperture is something which restricts the diameter of the light path through one plane in an optical system. The aperture stop or simply the stop is the limiting aperture—the aperture which restricts the diameter of the cone or cylinder of light that can enter and pass through. The diameter of the aperture stop is sometimes simply referred to as the aperture of the system. Note that the aperture stop is not necessarily the smallest aperture in the system. Magnification and demagnification by lenses and other elements can cause a relatively large aperture to be the stop for the system.
The aperture stop of a lens can be adjusted to control the amount of light reaching the film or digital sensor (CCD or CMOS). In combination with variation of shutter speed and film speed, the aperture size regulates the film's degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.
A device called a diaphragm controls the aperture. The diaphragm can be considered to function much like the pupil of the eye—it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (and the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.
Aperture is usually measured in f-numbers. Lenses have a set of "f-stops" that represent doublings in the amount of light let through the aperture. A lower f-stop number means a greater aperture opening which allows more light to reach the film. A typical lens will have an f-stop range from f/16 (small aperture) to f/2 (large aperture) (these values are approximate and may vary). Professional lenses can have f-stops as low as f/1.0 (very large aperture). These are known as "fast" lenses because they allow much more light to reach the film and therefore reduce the required exposure time. Large aperture prime lenses (lenses which have a fixed focal length) are favored especially by photojournalists who often work in dim light, have no opportunity to introduce supplementary lighting, and capture fast breaking events.
Zoom lenses typically have a minimum aperture of f/2.8 to f/6.3 through their range. A very fast zoom lens will be constant f/2.8, which means the aperture will stay the same throughout the zoom range. A normal zoom will be a constant f/4, and a consumer zoom will typically have a variable diaphragm, normally being something along the lines of f/4.5 to f/5.6, or even f/4.5 to f/6.3 (rare). There are a few exceptions to this rule, as even high quality hyperzooms often have as slow of an aperture as f/5.6 throughout the whole zoom range. Such is the case with most lenses which have more than 4x zoom range, like a 100-400 mm f/5.6.
The reason for consumer zooms to have a variable aperture is that the f-number is proportional to the ratio of the focal length to the diameter of the diaphragm opening. This means that if you have a 75-300 mm lens, a physically bigger diaphragm opening will be needed at 300 mm than at 75 mm, to maintain the same f-number. More light is needed as the focal length increases, to compensate for the fact that light from a smaller field of view is being spread over the same area of film or detector.
Aperture priority refers to a shooting mode used in some semi-automatic cameras. It allows the photographer to choose an aperture setting but allow the camera to decide the correct shutter speed. This is sometimes referred to as Aperture Priority Auto Exposure, A mode or semi-auto mode.
The aperture stop is an extremely important element in most optical designs. Its most obvious function is to reduce the amount of light that can reach the image plane, to prevent saturation of a detector or overexposure of film. The aperture stop has far more important functions, however:
- The size of the stop is one factor that affects depth of field. Smaller stops produce a longer depth of field, allowing objects at a wide range of distances to all be in focus at the same time.
- The stop limits the effect of optical aberrations. If the stop is too large, the image will be distorted. More sophisticated optical system designs can mitigate the effect of aberrations, allowing a larger stop and therefore greater light collecting ability.
- The stop determines the system's field of view.
- The stop determines whether the image will be vignetted. Larger stops cause the intensity reaching the film or detector to fall off toward the edges of the picture.
Maximum and minimum aperturesEdit
The specifications for a given lens might include the minimum and maximum apertures. These refer to the maximum and minimum f-numbers the lens can be set at to achieve, respectively, the minimum and maximum input of light. For example, the Canon EF 70-200mm lens has a maximum aperture of f/2.8 and a minimum aperture of f/32. This may seem counterintuitive since the maximum aperture has a smaller number while the minimum aperture has a larger number, but makes sense since the smaller number corresponds to a physically larger aperture opening. This can be remembered by thinking of the f/numbers as fractions and recalling that 1/2.8 is greater than 1/32.
The maximum aperture tends to be of most interest (makes it easier to shoot under dim lighting conditions, or at fast shutter speeds because the lens lets more light through to the film or CCD) and is usually included when describing a lens (e.g., 100-400mm f/5.6, 70-200mm f/2.8).
The minimum aperture is useful for time-lapse pictures shot on film (it places an upper limit on the exposure time for a given lighting condition) and maximum depth of field.