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Re: [OM] OM1 meter color sensitivity

Subject: Re: [OM] OM1 meter color sensitivity
From: "Peter A. Klein" <pklein@xxxxxxxxxx>
Date: Mon, 02 Nov 1998 01:46:10 -0800
At 05:30 PM 11/1/98 -0500, Perry Bain wrote:

>I recently got an orange filter for B&W photography, thinking it would be a
>good compromise between a yellow and red. I've always just metered through
>the filters on my OM1, expecting that the in-camera meter would correct for
>the decreased light. While using this filter, though, I didn't seem to be
>losing as much light as I thought I should (I was expecting maybe 1 to 1.5
>stops). In fact, the meter indicated less than half a stop change when I
>put the filter on. (BTW, this is a Hoya HMC O (G) filter.)
>
>This leads me to the conclusion that perhaps the OM1 meter is primarily
>sensitive to light at the orange/red end of the spectrum. Does anyone else
>have any experience with this? I'm thinking that perhaps some adjustment of
>the ASA dial may be needed with this type of filter.
>
>Perry Bain
>bainp@xxxxxxxxxxxxxxxxxxx

Perry:  The OM-1 meter and the OM-2 manual meter use CdS photoresistors.
As you've observed, CdS cells are more sensitive to the red end of the
spectrum than the blue-green range (the opposite of B&W film). This leads
to exactly the effect you describe.  Your best bet is to meter without the
filter and apply the correction factor.  I believe the standard factor for
an orange filter is 3, or 1.666 stops.

You could also make some bracketed test shots under typical conditions and
figure out what factors to apply to your meter readings with the filter.
The ideal filter factor will vary with cloud conditions and time of day.
There's much more red in sunlight in the first and last hours of the day.
Shadows illuminated by blue skylight are going to be darkened, but not if
illuminated by white cloud reflections.  After a couple of rolls of your
standard film using the G filter, you'll get a feel for it.

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.

One other thing to remember--the orange filter darkens green foliage and
vegetation quite a bit.  That's fine if you want it that way.  But if you
want to darken the sky to bring out clouds, but also like foliage and
vegetation more luminous, try a green or yellow-green filter.  These
filters also give more natural skin tones than the orange filter.

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?
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