True exposure
True exposure
The eective shutter speed for a central lens shutter is dened
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 inuence on the eective shut-
ter speed as the lens aperture closes to its setting. The faster
the shutter opens and closes, the less this inuence will be. It is
also follows that the inuence will be greater on shorter shutter
speeds.
With the lens at full aperture (largest opening), the amount of
light at the "lm plane" appears as illustrated by the blue curve in
the diagram. The eective 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 eective 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 er-
ror 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 illus-
trated 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 aper-
ture combination cannot be adjusted by True exposure.
When using ash/strobe as the main light source
and using the 1/800s shutter speed, you should
turn o the True Exposure facility (Custom Option
#13). Failure to do so will cause underexposure.
You can download a full explanation of this
situation from www. hasselblad.com.
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