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Disparity between filter factor and meter reading


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#1 Brian Rose

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Posted 23 February 2008 - 12:00 PM

All,

I'm doing a camera setup that involves various colored filters, and I noticed a funny thing with my Wratten Red 25. The stated filter factor is 8, or three stops. But, when I use my spotmeter, both with and without the red filter in front, the difference between readings was only a stop and a half. I repeated this test with my Spectra pro, as well as a third meter, and always the same results. Three meters can't all be wrong, but I can't believe that Kodak would be a stop and a half off in their filter factor, either? Anyone ever encountered this before, or have any idea why such a disparity?

Brian R.
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#2 David Mullen ASC

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Posted 23 February 2008 - 12:47 PM

All,

I'm doing a camera setup that involves various colored filters, and I noticed a funny thing with my Wratten Red 25. The stated filter factor is 8, or three stops. But, when I use my spotmeter, both with and without the red filter in front, the difference between readings was only a stop and a half. I repeated this test with my Spectra pro, as well as a third meter, and always the same results. Three meters can't all be wrong, but I can't believe that Kodak would be a stop and a half off in their filter factor, either? Anyone ever encountered this before, or have any idea why such a disparity?

Brian R.


Maybe they are compensating for the loss of sensitivity of the film dealing with such a red image -- in other words, if you didn't compensate for the filter, you'd notice a grey scale ended up three-stops underexposed on film, even though the density of the filter was less.
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#3 Jonathan Bowerbank

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Posted 23 February 2008 - 12:50 PM

We're you reading an 18% grey card? If not, could it be possible the surface you were reading might read higher on the grey scale once filtered red? Just as caucasian skin tones, when shooting b&w and using a red filter, will show up milkier and whiter.
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#4 Brian Rose

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Posted 23 February 2008 - 01:08 PM

Thanks for mentioning that. I did use a graycard, and still the same reading. And when I use a Blue 47 and a Green 61 (I'm doing tricolor work), my reading versus the FF is just about dead on. It's only the 25A that is off, which leads me to suspect that what David mentioned is likely the source of the disparity. So if that's the case, should I ignore the meter reading, and stick with the manufacturer's recommendation? I'm doing test stuff right now, so I can probably just do both and see what happens.

Thanks for the great tips!
Best,
BR
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#5 Hal Smith

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Posted 23 February 2008 - 02:17 PM

Just a thought here: Compare the filter's red spectral bandwidth graph with the red layer sensitometric data of the film you're using. If the filter is narrower than the film then with the filter less total red is getting through to the film. Kodak's filter factor may be addressing this concept.
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#6 Brian Rose

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Posted 23 February 2008 - 03:35 PM

Thanks a lot. I should have done that sooner, but I did consult the tables for the particular film, and I think you and David are right. And since these are tests, I may just do a variety of exposures, and see which ones come out best. Thanks!
Best,
BR
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#7 Chris Keth

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Posted 23 February 2008 - 11:38 PM

Lightmeters have a spectral response just light film does. If your meter is more sensitive to the blue end than the red end, a red filter would make a greater difference in its reading than if your meter's response was completely linear.
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