The cell which gathers light for otf has an output whereas cds cells change
resistance based on light. The OM-4 cell gathers light from the first
curtain/film
unitl the circuit reaches a quench point for 18%. This is when the magnet is
released (it had been preventing the 2nd curtain from moving) and the second
curtain enters the film plane.
Very bright light quenches the cell quickly so you have high shutter speed up to
1/2000.
Low light takes longer to quench, so you have a long shutter speed.
I don't think the circuit waits if it is low light or the ss is below 1/60,
the ss
is computed in real time as it is happening, whether it be 1/63 of a second or
1/40th.
John
C.H.Ling wrote:
> John Hermanson wrote:
> >
> > He's just about right. When you think about it, as the slit gets wider and
> > wider, the cell sees less and less of the curtain and more of the film.
> > The otf cells are active while the first curtain is moving so the circuit is
> > always blending curtain and film reflectance .
> > 1/60 would be the fastest speed where the film is totally exposed to the
> > cell
> > before the second curtain starts to move.
> >
> > John
> >
> Hi John,
>
> I want to go a bit further here:
>
> In auto mode.
>
> - the mirror up and SBC check for shutter curtain reflection.
> - if reading above 1/60s, computer fixed the shutter speed here.
>
> - if reading below 1/60s, no value are set.
> - when shutter is FULLY OPEN, another value from film reflection are
> checked.
> - film reflected light are accumulated and calculated by the computer,
> when exposure is enough, computer send signal to close the second
> curtain. (of course the computer should close the shutter a bit early to
> take the curtain travelling speed into account.)
>
> I think the computer will not perform any measurement during blending of
> film and shutter curtain. The above is what I think only, it may be
> wrong. But this seems a more logical design.
>
> C.H.Ling
>
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