> So far as I understand it he variation of the 'speed' of a focal plane
> shutter is obtained by varying the width of the slot in the blind of a
> shutter which, in fact, moves at a constant speed. It has been suggested
> that varying the width a slot in a gravity actuated shutter in front of
> the lens would not have the same effect of varying the speed. If this is
> so, why would the effect vary between the two types of shutter ?
Terry,
The following reply is not supported by being fully awake yet but I think that
it would have the same effect if the slot size in front of the lens is large
but when it starts to get to be the same width as the size of the lens
diaphragm or lens "entrance pupil" then I believe there are shutter efficiency
considerations that come into play.
Normally you can assume that exposure time is a function of the length of time
it takes the slot to travel its own width (this is true in a 100% efficient
shutter). However, with a slot that is close to the lens and is small compared
to the lens then the exposure time is a function more and more of the time it
takes the slot to travel across the lens and the decreasing size of the slot
simply cuts light level down but exposure time is not altered as muchb as
expected.
I hope I have sort of covered this OK and if not I hope someone who has had a
cup of coffeee already can confirm or correct my "explanation".
cheerio,
andy
Andrew Davidhazy, Professor
School of Photographic Arts and Sciences/RIT
andpph@xxxxxxx http://www.rit.edu/~andpph
