The Camera

A camera has a lens with a fixed focal length, which obeys the thin lens equationFocusing is achieved by moving the lens backwards and forwards relative to the film. As the object approaches the lens, so that the object distancedecreases andincreases, I over v must decrease and so v must increase. The image is formed further from the lens so the lens must be moved away from the film so that the image is formed on the film.

Besides adjusting the lens image distance, many cameras allow you to adjust the shutter speed and the f – stop. The shutter speed determines how long the shutter is open. Leaving the shutter open for longer allows more light to enter so the film is well exposed, but will mean the image is blurred if the object is not stationary or the camera is not fixed to a tripod. Unless a camera is mounted on a tripod, shutter speeds of 1/60 or faster are recommended. Typically the shutter is open for between 1/500 of a second and a few seconds.

The f – stop setting determines the size of the iris diaphragm in front of the shutter. On sunny days the opening is small to prevent overexposure of the film. The f – stop is a number that specifies the opening diameter:whereis the focal length andis the diameter of the diaphragm opening. Notice that a small D means a large f – stop number. Typical f – stop numbers are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22. These numbers have the property that the ratio of successive numbers is approximately the square root of 2. Since for a circular opening of radius the areaincreasing the f – stop number successively decreases the area of the opening to a half. If the shutter speed remains the same, the amount of light exposing the film will also be halved. The camera settings are then more easily adjusted for different light conditions. For example, a shutter speed ofs and an f – stop of 5.6 allows the same amount of light to enter as a shutter speed ofs and an f – stop of 8 or a shutter speed ofs and an f – stop of 11. Faster shutter speeds are preferred for moving objects. Also, large f – stops, corresponding to small openings, allow light only through the centre of the lens, improving the depth of focus so that near and far objects are simultaneously in focus.

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