Aha. Elementary physics triumphs over a technical dunce of a creative
photography teacher. As it should! You all nicely pointed out the
salient factors in this discussion and brought me back to the light.
And other people who wondered, as well, I hope. Nice little diversion
though. Thanks Per, Bob, Andy and the responses yet to come.
Don
ADavidhazy wrote:
Don,
I agree that the shutter opens from the center and that part is the last
covered on the return trip of the blades. However, consider this.
Imagine you
are standing in the center of the image plane with a light meter and a
friend
of yours is standing on the corner of the frame with a light meter. The
diaphragm aperture is set to f/5.6.
Now imagine that the shutter is closed. Neither of you will register
any light.
Now imagine that the shutter has opened slightly. Maybe to a point
where the
opening created by the blades is the equivalent of f/32.
Both of you at that time take an incident light meter reading. Note
that both
of you "see" or are illuminated by the same effective lens area. That
associated
with f/32. Both meters should read the same.
Now the shutter opens some more. Maybe to f/11. Both meters register an
increase in light level. It is 3 stops more than the level was at f/32.
Now the shutter opens some more. Maybe to f/5.6. Both meters register an
increase in light level. It is 2 stops more than the level was at f/11.
Now the shutter continues to open further until it gets to its maximum
opening.
The meters are still only registering the level associated with f/5.6
because
the diaphragm limits the amount of light that can pass through the lens.
Once fully open the blades stay there for a while depending on the
exposure time
set. Then they start on their return trip. Once they pass f/5.6 the
blades start
to diminish the effective aperture of the lens and gradually the light
level as
read by the meters drops, simultaneously, until all light is
extinguished.
Will there be a discrepancy between the readings between the two
meters metering
at the same time? Absolutely. But this is due to the fact that from
the corner of
the frame one is looking at the circular hole of the aperture from an
angle and the
nice round circle is not a circle anymore but an oval shape of smaller
area than
the full circle and so transmits less light towards the edges. Also,
note that the
distance from the center of the lens to the center of the frame is
less than that
to the corners, so more or less according to the inverse square law,
the light
level has to be less at the corners.
Focal plane shutters were not developed to correct for uneven
illumination at high
speeds but because they did offer high speeds and only one shutter was
needed if
ready and relatively inexpensive lens interchangeability was desired.
FP shutters
have their own problems, however, with providing an even exposure
across the frame!
more or less IMO, ;)
Andy
