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H4D
T
1
T
3
= T
1
T
2
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 shutter speed as the lens
aperture closes to its setting. The faster the shutter opens and closes, the less this inu-
ence 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 error since the behavior
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.
Note
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 explana-
tion of this situation from www.
hasselblad.com.
True exposure
Shutter position
Open
Closed
Time
Exposure at max aperture (e.g. f/2,8)
Exposure at aperture f/4
Exposure at aperture f/4 adjusted with true exposure