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filtering super-panchro film for good pictures


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#1 Dennis Couzin

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Posted 14 November 2014 - 09:07 AM

As fewer filmstocks are available, we must learn how to adapt to what there is.

Three B&W negative films called Fomapan, made by Foma in Czech Republic, may become available in 16 mm cine gauge.  Find Foma's data sheet for Fomapan 200 Creative here: http://www.foma.cz/en/fomapan-200

One peculiarity of Fomapan is that its spectral sensitivity is high out to nearly 700 nm. It might be termed "super-panchromatic".  There is nothing wrong with having too long spectral sensitivity, since what isn't needed can be filtered out, but the cinematographer will have to learn how to filter the super-panchro film for good pictures.

Human vision responds to the different wavelengths according to the photopic luminosity function.  Here it is graphed on a logarithmic scale appropriate to photographic sensitometry: http://www.mediafire...fmkd6/human.jpg .

B&W pictures are visual lightness records when the film's spectral sensitivity function matches the eye's.  Comparing the "human" curve with various published panchromatic spectral sensitivity curves shows that all the films are much too sensitive in the blue wavelengths than they should be.  That is why many B&W cinematographers routinely employ a yellow filter.  The K-2 yellow filter is the most popular.  For the red wavelengths the panchro films curves accord decently well with eye's.  

We unfortunately do not have proper spectral sensitivity curves for the Fomapan films.  Foma provides a curve which they call a "spectrogram".  Notice that their curve is lacking a scale on its vertical axis.  Calling the sensitivity "relative" does not exempt you from having this scale.  Since the spectrogram is made with a logarithmic density wedge, the scale will be logarithmic.  "Relative" means you don't care where you mark "0" on the vertical scale, but the scale must show how far it is from 0 to 1.  Otherwise you don't know how sensitive the film is at one wavelength versus another.  Foma would have to provide additional information:
    range of the neutral density wedge
    gamma of the film
    how the curve was picked off the spectrogram (continuous tone image)
in order to determine a scale for the vertical axis.  Better they just figure out the scale. 

I played around with Foma's curve and can at least estimate that the relative spectral sensitivity function for Fomapan 200 is somewhere between this:

http://www.mediafire...an200_sf1.3.jpg
and this:
http://www.mediafire...an200_sf2.0.jpg

The main problem with being super-panchromatic is recording red colors as too light.  It is the opposite of shooting with orthochromatic film.  Imagine a very red thing that reflects all wavelengths past 600 nm.  In D65 daylight, the human eye sees it as 15% as light as a white thing.  Unfiltered Fomapan sees it as 23% as light as white.  But Fomapan with the K-2 filter sees it as 43% as light as white.  Therein lies the problem.  The excessive red sensitivity in itself does not make reds photograph drastically light, but when the excessive blue sensitivity is corrected the reds photograph fully 1½ stops too light.

Color conscious B&W cinematographers will want to find the long wave complement to the K-2 filter to correct a super-panchromatic film such as Fomapan.  A strong Kodak CC Cyan-2 filter is in the ballpark, but Kodak has discontinued the Cyan-2, and anyhow gels are not practical for regular use.  A Schott KG-3 glass is in the ballpark, but you must find one that has not been tempered as "heat absorber" or it will be optically poor.  Perhaps there is a high quality anti-reflection-multi-coated "photographic" filter to roll off the sensitivity past 600 nm.  Good hunting.


Edited by Dennis Couzin, 14 November 2014 - 09:07 AM.

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#2 Dennis Couzin

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Posted 14 November 2014 - 10:19 AM

Oops!  One link in the previous post got an extra "http//".  Fixed here:

 

... the relative spectral sensitivity function for Fomapan 200 is somewhere between this:

http://www.mediafire...an200_sf1.3.jpg
and this:
http://www.mediafire...an200_sf2.0.jpg


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