True exposure
Shutter position
Open
Closed
T3 = T1 |
T1 |
T2 |
Exposure at max aperture (e.g. f/2,8) Exposure at aperture f/4
Exposure at aperture f/4 adjusted with true exposure
Time
True exposure
The effective shutter speed for a central lens shutter is defined as the length of time between the opening and closing when measured at the half height position when expressed in diagram form (see diagram). The fact that it will take some time to open and close the shutter will have an influence on the effective shutter speed as the lens aperture closes to its setting. The faster the shutter opens and closes, the less this influence will be. It is also follows that the influence will be greater on shorter shutter speeds.
With the lens at full aperture (largest opening), the amount of light at the film plane appears as illustrated by the blue curve in the diagram. The effective shutter speed then becomes T1. If the lens is now closed down by one stop, the amount of light appears as illustrated by the red dashed curve. The effective shutter speed is now increased to T2, which is longer that T1. The result is that the exposure is not reduced by exactly one stop (1EV), however, but slightly less. At the shorter shutter speeds, the exposure error can be as much as 0,5 – 0,8 EV.
The True exposure mode can compensate for this exposure error since the behaviour of the shutter is a known and predictable factor. At shutter speeds of 1/150 second or shorter (faster), the camera will shorten the shutter speed to compensate, as illustrated by the green dashed curve. At the fastest shutter speeds, however, it is not possible to adjust the shutter speed and so the aperture is adjusted instead.
Although it is probably an infrequently used combination, please note nevertheless that the fastest shutter speed / minimum aperture combination cannot be adjusted by True exposure.
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