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See Aperture priority exposure.
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An auto-exposure mode where you select the required lens aperture and the camera sets the necessary shutter speed, to give the correct exposure based on the auto meter reading. This mode is ideal for landscape and still-life photography where maximum depth-of-field is required. It's either indicated on the camera as AP (aperture priority) or AV (aperture value).
The lens focal length divided by effective diameter of the aperture gives the f/number that's used to indicate the aperture value. Each full f/number, also called a stop or f-stop, halves or doubles image brightness and some lenses can also be controlled in half or third steps. The most common f/numbers are 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22. They are usually preceded by an "f". The larger the f-number, the smaller the lens opening. In the aforementioned series of numbers, f/1.4 is the largest opening and f/22 the smallest. The smaller stops (larger f numbers) give the greater depth of field in a photograph, and vice versa.
The combined shutter and aperture values used on older cameras and light meters.
To make an exposure, a photographer can, for example, choose a combination of a small aperture and a slow shutter speed or a large aperture and a fast shutter speed. Each movement of the aperture is classed as one stop – go from f/8 to f/11 and you close the aperture by one stop. Similarly by adjusting the shutter speed from 1/125sec to 1/500sec you reduce the exposure by one stop. If you adjust the aperture by a stop and counteract this by also adjusting the shutter speed by one stop you will produce the same exposure value. Therefore an exposure of f/8 at 1/60th could be changed to any of the following combinations: f/11 at 1/30th, f/16 at 1/15th, f/5.6 at 1/125th or f/4 at 1/250th. This is called the law of reciprocity (if one value increases the other will decrease proportionally and visa versa).
Reciprocity law states that as you increase the intensity of light reaching the film you also need to decrease the speed it reaches the film by the equivalent amount. Most films work quite happily between exposures of 1/2sec and 1/1000sec, but go beyond these extremes with a very low intensity of light and a long exposure or a very high intensity of light and a correspondingly short exposure and the law fails an exposure increase may be required when the shutter speed is beyond these limits. At these extremes the law fails. Compensation is required to adjust for this, but there is no strict rule to correct the error. Most film and paper manufacturers provide technical details on request with a rough guide to exposure adjustments. As a rough guide for an exposure of one second you would increase the speed to two seconds, or open the aperture by one stop. A speed of 10sec would need to be increased to about 50sec or open the aperture up two stops. With black & white film you only have to worry about this exposure correction, but with colour film does not only suffer from exposure problems but also colour casts. A colour film is made up of three individual colour layers, each layer suffers from reciprocity failure at different levels. On an uncorrected film the shadows may have a magenta colour cast but the highlights may suffer from a cyan cast. To correct the cast not only would a longer exposure be needed but also the inclusion of a colour correction filter of a low value, care would have to be taken in choosing the correct filter otherwise an over corrected result may appear.