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Old April 2nd 04, 12:29 PM
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Default Focal plane vs. leaf shutters in MF SLRs

"Bob Salomon" wrote in message
In article ,
Lassi Hippelainen wrote:

because they can expose all parts
of the image gate the same amount of time.

Only at the synch speed or slower. At faster speeds only part of the
image is exposed at any one time as the shutter becomes a moving slit.
The higher the speed the narrower the slit.

This is why you have limited flash synch with a focal plane shutter vs a
leaf shutter.

You didn't get what he wrote which was "because they can expose all parts
of the image gate the same amount of time." He didn't write "at the same
time". He wrote "the same amount of time". What he was pointing out is
perhaps the most important feature of leaf shutters is that because they
exist near the lens aperture diaphram then they work in the same way at
reducing the light. If you reduce the aperture then the image doean't go
dark at the outside and towards the centre as you reduce it. It gets darker
the same amount all over the frame. So if the leaf shutter is there it does
not matter how long it is obscuring the edge of the image in relation to the
centre of the image. No matter how the blades are working, the effect is
spread over the whole image. So the centre does not get more exposure than
the edges, even though the centre of the blades is open for longer than the
edges. So if the leaf shutter were at the back near the film plane then the
centre of the image would get exposed more than the edges of the image due
to the way the blades have to open and close. You would need an extremely
fast-acting blade system to overcome this so that the time in transit for
the blades was negligible compared with the time the blades were fully open.
But a focal plane cloth shutter gives regular exposure over the whole of the
area. The film edges get as much light as the centre as the slit moves
across. Each part of the film gets exposed for the same amount of time (not
"at" the same time). This is probably the most important reason why a leaf
shutter can not be used in this way and why it has to exist in the lens near
where the diaphram is.