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    Sometimes the combination of two values are required to provide a set result. On a camera the combination of shutter speed and aperture provide a certain amount of exposure. The total amount of exposure required depends on the amount of light available. A semi automatic camera can detect the amount of light available and then require the user to input either the shutter speed or the aperture. Some older cameras are shutter speed priority - the user enters a speed, and the aperture is calculated. Other cameras are aperture priority.
    An equal opportunity camera allows the user to enter either value and the alternate is calculated automatically. On a camera with a dial for each, it is required that one or other is set to automatic.

    The main failing on this system is that it would be nice for the user to be able to switch from aperture to speed, without losing the current speed value, which has been calculated by the camera. This would give the user a better starting point for their choice of speed. Unless the dials are motor controlled this is not possible. Motor control of dials in a device as compact as a camera is very unlikely. If the values were set with a row of LEDs, possibly visible through the view-finder then more options would be available.

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