Seems that you tested the filter factor from essentially neutral
coloured samples. While a certain shade of grey (13% or 18% depending
on equipment calibration and personal preference) is the result of an
average scene integrated to a nominal average brightness, the result of
a colour integration of an average scene is not grey. It is a murky
brown as far as I recall from my colour lab days, over 20 years ago.
Film spectral sensitivity and meter cell spectral response are also
factors in the final result (correct compensation). If the film you use
has extended red sensitivity then it would need less compensation than
the filter manufacturer suggests for red and orange filters. This is
why film data sometimes disagrees with the filter factors published by
the filter makers. Meter cells can be colour biased and can run into
trouble with strong colour shifts such as filtered light or very low
wattage tungsten lights.
Lastly, a scene can comprise a combination of illumination types, so
that the position of the subject matter in the whole scene has to be
considered. Daylight scenes have shadows that are mostly lit by the
open sky, so on the clearest of days the shadows are bluest. I always
found that the shadows of yellow filtered photos were too dark for my
taste and called for the film to be under-rated and developed less. But
these days I prefer green filters which I believe give the best balanced
brightness of all colours although some caution is needed for portraits
to avoid the bronzed tanned look (unless that's the desired effect, of
course).
Regards,
Anthony Farr
----- Original Message -----
From: "Shel Belinkoff" <[EMAIL PROTECTED]>
> 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.
> --
> Shel Belinkoff
> mailto:[EMAIL PROTECTED]
> -
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