Re: FED shutter profile?

[Joel.Alpers@xxxxxxxxxxx]

John B writes:

#
#Bob brings up an interesting point and that is that it would be nice
#to be able to fool the FED into giving the right f/number.  To start
#on a problem like this, it would be nice to know the time/opening profile
#for the FED shutter at different scene light levels.  Seems like Joel or
#the reclusive 8-) Marty could tell us.  Is the period of the thing 1/30
#second regardless and the leaves just hit different mechanical stops as 
#they open?  Or does hitting a closer mechanical stop shorten the period?  
#Or..?
#

In the FED, you really cannot separate aperture and shutter speed,
in the auto mode. The arrangement is an "iris" that is both - it opens
"a little or a lot", and unlike a separate iris/shutter, the opening/
closing time may be a significant part of the total time. For example,
in a high light situation the exposure curve might look like this:

  

           _---_	           <- Opens to perhaps f/11
   ______-~     ~-__________


While in a low-light, it is more like:

            _-----_            <- Opens to perhaps f/5.6
          -~       ~-
   _____-~           ~-________


So _both_ "aperture" and the "shutter speed" have increased.
This is a simplification, but you should get the idea. I've seen (but
don't have a copy of :( the response curve of shutter vs aperture -
both increase pretty linearly. Note that I've tried to sketch that
the opening/closing time _can be_ a significant part of the total
time.

What it boils down to is that the system, in auto-exposure mode,
won't give you small apertures at slow shutter speeds.

The fixed 1/30 you mention is quoted for flash modes, in which you
dial the aperture on the camera, and it fires - the same merged
shutter/iris is still in effect, so there is _some_ change
in "shutter speed" present, but at 1/30, the "full open" time
dominates.

Hope this helps!


Joel Alpers
Rocky Mountain Memories - Equipment and Supplies for the 3D Photographer
rkymtmem@xxxxxxxx
http://www.frii.com/~rkymtmem


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