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User Manual - Snap Studios

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True exposure<br />

Shutter position<br />

Open<br />

Closed<br />

T 3 = T 1<br />

T 1<br />

T 2<br />

83<br />

Exposure at max aperture (e.g. f/2,8)<br />

Exposure at aperture f/4<br />

Exposure at aperture f/4 adjusted with true exposure<br />

True exposure<br />

Time<br />

The effective shutter speed for a central lens shutter is defined as<br />

the length of time between the opening and closing when measured<br />

at the half height position when expressed in diagram form (see<br />

diagram). The fact that it will take some time to open and close<br />

the shutter will have an influence on the effective shutter speed as<br />

the lens aperture closes to its setting. The faster the shutter opens<br />

and closes, the less this influence will be. It is also follows that the<br />

influence will be greater on shorter shutter speeds.<br />

With the lens at full aperture (largest opening), the amount of<br />

light at the film plane appears as illustrated by the blue curve in<br />

the diagram. The effective shutter speed then becomes T1. If the<br />

lens is now closed down by one stop, the amount of light appears<br />

as illustrated by the red dashed curve. The effective shutter speed<br />

is now increased to T2, which is longer that T1. The result is that<br />

the exposure is not reduced by exactly one stop (1EV), however,<br />

but slightly less. At the shorter shutter speeds, the exposure error<br />

can be as much as 0,5 – 0,8 EV.<br />

The True exposure mode can compensate for this exposure error<br />

since the behaviour of the shutter is a known and predictable factor.<br />

At shutter speeds of 1/150 second or shorter (faster), the camera<br />

will shorten the shutter speed to compensate, as illustrated by<br />

the green dashed curve. At the fastest shutter speeds, however, it<br />

is not possible to adjust the shutter speed and so the aperture is<br />

adjusted instead.<br />

Although it is probably an infrequently used combination, please<br />

note nevertheless that the fastest shutter speed / minimum aperture<br />

combination cannot be adjusted by True exposure.

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