Re:cds cells and such

[P3D Michael Kersenbrock <michaelk@xxxxxxxxxxx>]

> Positioning of the laser and sensor would then be critical to accurate
> measurement but if you're trying for a greater accuracy anyway, this makes
> sense. I'm guessing but I think perfectly centered on the lens would be the
> most accurate.

That could be tricky (it seems).  The FED's automatic mode would be a good
example, assuming it works the way it's said to (can't tell by just looking
at it :-).  It's said that it's never "open", it's either in the process of
opening or in the process of closing (or closed).  It supposedly doesn't
"pause" open.   At a camera's fastest shutter speed, should the open/close 
transfer times be significant in respect to the overall time (as in the case 
of the FED, as reported), it seems that a light intensity integrator 
would be most useful to make a measurement. Seems that light through the center
would be "too long" in relation to the net effective exposure.  Especially with
fstops where the lens is set to be wide open.

Something like a light sensor operating in a linear fashion as a current source
charging a capacitor might work better.  Or something like that (for digital readout).
Alternatively using a storage 'scope (or equivalent) and then "measuring" the area
covered to calculate a square wave equivalent?

It seems that for slow shutter speeds (except possibly for the FED) it shouldn't 
matter much how it's measured, but for top speeds or weird shutters, it might
be tricky if one really wants accuracy.  Assuming it can be defined exactly. :-)

It also seems that effective shutter speed might be a function of f-stop
setting in some cameras with slow irises (like 50's stereo cameras 
tend to have, even if they weren't originally that way :-).

It's late Friday, I kind of hope I'm not making sense.... seems icky.  :-)

Mike K.


------------------------------