Not wanting to prolong this thread, but what effect does the number
of leaves have? It takes quite a few to approximate a circle over
the full range. I know that shaping the individual leaves can
improve the approximation, but there must be limits. Then again,
maybe it's just not important.
Further, it seems to me that Andy's argument requires some
assumptions about the lens that may not be justified -- e.g.
uniformity over the diameter of the lens from the center to the edge.
Roger
On 1 Nov 2007, at 5:49 PM, Don Roberts wrote:
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
