![]() Couple that with the fact that exposures are now broken down into thirds of a stop, trying to explain it all seems an exercise in futility. Since you don’t have to manually change both factors of an exposure with modern cameras new photographers that have a hard time understanding this relationship. If you are in aperture priority and change the f stop the shutter speed automatically changes for a proper exposure if you are in shutter speed priority and change the shutter speed the f stop automatically changes for a proper exposure. It is possible to have the same exposure with a variety of different f stops and shutter speeds depending on what effect you want to achieve. Modern cameras automatically do this for you. Each full f stop either halves or doubles the amount of light entering the camera and each full shutter speed stop either halves or doubles the amount of time of the exposure. Now if you look carefully you’ll see a relationship between f stops and shutter speeds. Photo by David Trainer ISO 400, f/5.0, 1/400-second exposure. In the old days you had to fudge between the click stops for a more accurate exposure. Now the reason both f stops and shutter speeds are broken down into thirds of a stop is to allow for a more accurate exposure. Modern cameras shutters are now calibrated in thirds of a stop so you have shutter speeds like 1/30, 1/40, 1/50, 1/60, 1/80, 1/100, and 1/125th of a second. These represented halving the amount of light one direction and doubling it the other. Each stop was either twice as long or half as much time. In the old days (again) each shutter speed was a click stop. Just like with the lenses “stops” the shutter also has stops. Where the job of the lens is to measure and focus the light the job of the camera is to open and close the shutter and record the image. The longer the shutter is open, the longer the light has to expose the film, the shorter the shutter is open, the less time there is to exposed the film. The other part of an exposure is the amount of time that the film or sensor is exposed. Photo by Thomas Hawk ISO 100, f/16.0, 20-second exposure. Using a plumbing analogy, a ¼ inch pipe will allow more water through it than an 1/8th inch pipe as long as the water pressure is the same. ![]() If you think… a hole that is 1/8th the length of the lens is smaller than a hole that is ¼ the length of the lens. One of the confusing parts of f stops is the larger the number the smaller the opening. Remember, an f stop is a factor and the bottom part of a fraction. F 8 means the hole in the lens is 1/8 the length of the lens. F 4 means that basically the hole on the lens is ¼ the length of the lens. Now, most camera lenses are calibrated into thirds of a stop so instead of f stop numbers of 2.8, 4, 5.6, and 8 (which are full stops) you have stops like 4, 4.5, 5, 5.6, 6.3, 7.1, 8 (which divides each stop into thirds. You could look through the lens and see the iris opening and closing as you rotated the dial. In the old days each stop was a click on the lens. Each “stop” either doubles or halves the amount of light allowed through the lens. It is a factor of these three things, so the name f (factor) stop. This f stop is determined mathematically by the size of the iris opening of the lens, the lenses focal length, and the dimensions of the film or sensor. The f stop (aperture) is the iris in the lens that allows a measured amount of light to strike the film. Photo by Susanne Nilsson ISO 100, f/9.0, 1/250-second exposure.
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