[rollei_list] Re: Question about Rollei Filters and compensation
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 28 Nov 2005 17:24:26 -0800
----- Original Message -----
From: "Don Williams" <dwilli10@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, November 28, 2005 4:59 PM
Subject: [rollei_list] Re: Question about Rollei Filters and
compensation
At 03:37 PM 11/28/2005 -0800, you wrote:
Peter.
No, according to Mannheim, the Rollei Mittel-Gelb has a
factor
of 3, the orange has 6. or to put it in EV terms, reduce
the EV
by 1.5 and 2.5 respectively.
The numbers on the Rollei filters refer to EV settings, not
filter factors.
As Don Williams suggested, put the filter in
front of your meter cell. Works for me.
It also works underwater, even for color film. I
use a CC20R or CC30R filter in my Rolleimarin,
and I have made up laminated (waterproof) filters
from the same gel stock for use with my light
meter. This really helps kill the blue-green
down to maybe 20-30' in clear water. Beyond that
there isn't enough red-end light to make an
available light exposure, so artificial lighting
is in order. (My Rolleimarin filters are
self-made, using thin flat glass blanks, clear
epoxy, Wratten gels, and a vacuum system which
assures no bubbles in the assembled sandwich.)
Jerry
Something that has always puzzled me about filter factors
is what, exactly, is being corrected.
For instance, suppose one is using a red filter to
photograph red objects. A red filter usually has a very
large filter factor, will not the objects be overexposed?
The transmission curves for the filter will show its
attenuation in the pass band. My instinct is that the
correction should be no more than this loss. For instance,
the transmission curves for a No.25 (wratten A) filter is
about 98% is its pass band. If we want normal rendition of
relative brightness of red objects should we not use
something close to the normal exposure? The idea of filters
is that they suppress colors in the stop band. Presumably,
we _want_ these colors to be underexposed but the desired
colors to be normally exposed.
I may be missing something obvious here such as
maintaining normal exposure of objects of mixed color but
changing their relative tonal value on the negative. I think
this is the case but it suggests that the filter factor is
no more than a sort of average guide and may be wrong for
some types of objects of scenes.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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