Peter Klein wrote:
>Now, if you used an OM-2 or later in auto mode with filters, things gets
>even more interesting. The silicon blue cells used in auto mode are more
>blue-sensitive and less red-sensitive than CdS cells. Much more like B&W
>film. I've read that you can use silicon blue cell readings without
>worrying about filter factors. But I'd still test it first. And you have
>to remember, the CdS cells are telling you one thing before you shoot, and
>the silicon blue cells are doing quite another once they take control.
More interesting still is the fact that my OM2-SP and OM4 exhibit the same
effect, and they use the same silicon cell to do both the metering display
and the auto-exposure calculation! I generally set a manual exposure
compensation when using orange or red filters, of +2 and +3 stops
respectively. I'm a bit puzzled as to why these bodies don't account for
the filter factor properly. Your explanation of the CdS' greater red
sensitivity is correct, but it seems that the OM silicon cells are very
similar in their spectral response.
>Hope this helps! Now, does anyone know why Olympus used CdS for the prism
>meter and silicon blue cells for the off-the-film automatic metering?
I'm only guessing, but since the OM-2 is derived from the OM-1 it was
probably easier for the designers to use the same meter in both. But CdS
cells are nowhere near fast enough for use in OTF metering. We use them in
our enlarging meters where sensitivity is more important than speed - it
can take a couple of seconds for the readings to stabilise in very low
light levels which is not much use in an auto-exposure camera!
Cheers
Richard
_____________________________________
Richard Ross, B.Sc, Ph.D, LRPS
RH Designs - innovative darkroom equipment
http://www.nildram.co.uk/rhdesign
tel: +44 (0) 1442 258111
richard@xxxxxxxxxxxxxxxxxxxxxxx
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