----- Original Message -----
From: "Shel Belinkoff"
Sent: Sunday, September 16, 2001 12:43 PM
Subject: Filter Factors
> Everything I've read recently, including the information sheet
packed
> with my new Pentax SMC filters (my thanks to those who helped
me get
> these great filters) suggests that the filter factor for a
Medium
> Yellow filter (Y2) is 2X, or 1 stop. So, while calibrating my
gear
> this morning I decided to check the needed exposure
compensation using
> two Pentax spotmeters, the digital Zone VI modified meter and
the
> standard, but calibrated, Spotmeter V. Both showed that the
needed
> exposure compensation would be about 1/3 stop more with the
filter
> than without. I metered off a white truck, the overcast sky,
and a
> building. Always 1/3 stop. Off some other colors there was a
slight
> difference in the readings between the Zone VI meter and the
standard
> meter, which is to be expected, but in no circumstance did
either of
> the meters indicate the need for an additional full stop with
the Y2
> filter.
>
> I suspect this discrepancy may be due to the sky being
overcast and
> there being less blue light for the filter to absorb.
>
> However, using the meter on the LX, I'm shown a 1-stop
compensation.
> But the LX meter, and other built-in meters, don't show
fractions of
> exposure readings, even though the camera may actually make
the
> exposure between one and another shutter speed. Fr example,
I've used
> the LX to make a normal exposure, and then used exposure
compensation
> to increase or decrease the exposure by 1/3 stop. Sometimes
there was
> a change in the readout, other times not, depending, I assume,
on how
> close the difference in exposure was to the point where it
would show.
> So, it seems, using the meters built into the camera, while
giving
> accurate exposure, won't necessarily tell me what the
compensation
> was.
>
> Since I'm going to be some work with hand-held meters and
manual
> cameras, can we discuss this situation a bit. One thought
that
> crossed my mind was to make the exposure readings off a grey
card,
> which I'll try later when I'm outside and away from the
computer. Any
> thoughts on that idea.
One of the problems with the LX meter (and likely most meters
from the era) is that they are not colour linear. The LX is
known to be oversensitive to yellow to red light. While the Y2
filter isn't that strong, it is cutting a significant amount of
blue, so I expect this is where the 1/3 stop discrepancy is
coming from.
I don't think it is possible to get a really accurate meter
reading from any built in camera meter when metering through a
coloured filter because of meter non linearity. All of my
cameras, including the newer MZ-5 shows severe meter innacuracy
when metering through coloured filters. I would trust the
modified meters 1 1/3 stop reading over the manufacturers filter
factor of 1 stop.
Fortunately, the Zone VI modified meter eliminates this problem.
Your calibrated but not modified meter will display the problem
with stronger coloured filters than the Y2 (I know this from
prior experience).
William Robb
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