Re: leaf shutters

[dlevy@worldy.com]

Not sure that I agree, if I understand what is being said. I have used both
Voigtlander and Ziess Ikon cameras of different types over the past several
years (decades) in general concept, Zeiss tended to place the shutter
between the lens behind the aperature (iris). Voigtlander, place the shutter
aft of the lens. Typical of these designs are 2 of their 35mm cameras near
and dear to my heart; the Ikonta 35 and the Vito B. Based on what you said,
I should experience more fall off with the Vito than with the B, but it is
just the opposite. In fact generally I have found the Zeiss models have more
fall off. Of course this is far from a controlled experiment due to lens
design, but the lens and shutter I think must be taken as a whole in this case.

By there very design a leaf shutter is open longer in the center than at the
edges since it opens from the center outward. There is the inertia required
to open, stop, and close which should create a rather nonlinear action.
Possibly in designing the shutter an engineer could use these actions to
even out to some extent the time difference between the center and edge, but
I can't envision it being done.

I have seen reference to this in more than one publication, and one on lens
design printed in I think the 30s which I borrowed had a whole chapter on
this design limitation including diagrams, formula, etc. It went through the
whole thing including the explanation of the inertia problems beyond 1/500th
of a second. Unfortunately, I don't remember the book's name or which friend
I had borrowed it from. 

Since there are flaws in both focal plan designs and leaf shutters, it all
is probably academic, unless we devise a better system or go back to the hat
over the lens shutter.
 
 a rather At 09:18 AM 11/19/96 -0500, you wrote:
>    There has been some comment about leaf shutters not exposing evenly,
>i.e., less exposure at the edges of the frame than in the center.
>    There was a very good (IMHO) discourse on this recently on one of
>the rec.photo newsgroups by one David Rosen, and what I say below is a
>summary paraphrase of him. Makes sense to me, but what do I know? So,
>*FWIW*: 
>   The accusation is not *usually* so, in that leaf shutters are
>commonly located adjacent to the iris. Such BETWEEN-the-lens shutters
>can be considered a second iris, albeit one that closes all the way and
>has a timing mechanism governing its opening/closing. So why doesn't the
>*real* iris also cause uneven exposure? Because both are located at or
>near the convergent point of all light rays entering the lens. In fact
>some old b-t-l shutters were also the iris, in that their *amount* of
>opening was controlled by the iris setting (there being no separate
>iris).
>    The issue of the proportion of total exposure time consumed by the
>blades in motion, opening and closing, affects the effective shutter
>speed. The marked times on shutters are often a fudge of theoretical and
>effective speeds.
>    True, BEHIND-the-lens leaf shutters are a different matter. Uneven
>exposure is  (at least theoretically, and possibly actually) possible
>with this location.
>
>Stan Yoder
>Pittsburgh
>
>    
>
>
Brian Levy, J.D.
Agincourt Ont.
dlevy@worldy.com