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Old November 18th, 2012, 06:31 PM
Nick Masson Nick Masson is offline
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Default Light metering with red 25A filter

Hi All,
I am seeking some advice when it comes to using a 25A red filter with BW film... I recently returned from a trip to Uganda, and chose to try out a newly acquired 25A red filter. The specs specify that one should stop down 3 stops when using the red filter, yet I found some of my negatives to be slightly over-exposed. As with all photography, I probably just need more time to experiment and learn how to read my environment better (i.e. anticipate results with a red filter), but if anybody has advice I would appreciate it.

fyi. I am not metering through the lens, and I typically meter off of the foreground to put my 'non-sky' region around middle-grey. As I understand it, the 25A will drop the foreground about 3stops, wheras the sky will drop an additional 2stops (net 5).

Most of the shots that looked somewhat overexposed had bright green foliage and rich, red soil, so perhaps there wasn't as much of a 'blue cast' in my foreground scenery as I might typically expect if I were photographing, say, a pine forest in Colorado. Thus I may have had more light transmission than anticipated from my foreground when metering off of the foliage/soil and dropping three stops...

I got more predictable results with the polarizing filter (objective being to pull out the clouds), so maybe i'll stick with that until I get a better hang for the 25A.

Thanks!
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Old November 19th, 2012, 04:47 AM
Bart_van_der_Wolf Bart_van_der_Wolf is offline
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Quote:
Originally Posted by Nick Masson View Post
Hi All,
I am seeking some advice when it comes to using a 25A red filter with BW film... I recently returned from a trip to Uganda, and chose to try out a newly acquired 25A red filter. The specs specify that one should stop down 3 stops when using the red filter, yet I found some of my negatives to be slightly over-exposed.
Hi Nick,

I'm sure you meant to say open up 3 stops, because the filter blocks most of the light for which panchromatic film is sensitized. Only wavelengths longer than 580 nanometres pass a bit more freely, but then as you get closer to 700 nm you will start to experience the particular Red/near-IR cutoff of your film material.

Then, besides the reflected spectrum by your subjects (and there may have been quite a bit of Near Infrared contribution as well, if your film was sensitive to that) and the actual band-pass between 580 and 700 nm, your light meter may not be as sensitive to this extreme end of the visible spectrum as for the rest of the wavelengths you normally use to meter of.

Quote:
As with all photography, I probably just need more time to experiment and learn how to read my environment better (i.e. anticipate results with a red filter), but if anybody has advice I would appreciate it.
Yes, a bit of experimentation is indeed inevitable, but for a given type of subject the results should then be pretty repeatable. It's just the effect of the combination of variables that needs to be worked out. A good thing about negative film is that it has some overexposure latitude, so hopefully your shots are still usable.

Quote:
fyi. I am not metering through the lens, and I typically meter off of the foreground to put my 'non-sky' region around middle-grey. As I understand it, the 25A will drop the foreground about 3stops, wheras the sky will drop an additional 2stops (net 5).

Most of the shots that looked somewhat overexposed had bright green foliage and rich, red soil, so perhaps there wasn't as much of a 'blue cast' in my foreground scenery as I might typically expect if I were photographing, say, a pine forest in Colorado. Thus I may have had more light transmission than anticipated from my foreground when metering off of the foliage/soil and dropping three stops...
Indeed, and combined with the spectral sensitivity of the lightmeter, that may be the explanation.

You could try metering through the filter, and see what that suggests. When you point towards white clouds, you will have a pretty good spectral range of illumination. Maybe that will reveal a bit about the lightmeter's spectral sensitivity curve, when you compare a reading with and without the filter.

When I still shot B/W film, many moons ago, I sometimes used a better copy (than this scanned copy of a printed version) of a 'Lagorio chart' like this specimen:


The original has a line which matches between the visual brightness of the color and a grayscale. When photographed with B/W film it helped to determine the mismatch between the film and human vision, and it was helpful in quantifying the effect of filters.

Cheers,
Bart
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Old November 19th, 2012, 08:01 AM
Nick Masson Nick Masson is offline
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Thanks for the explanation Bart. and yes, I meant open up not stop down, sorry. I guess not very many people use film anymore, but I wonder (perhaps you know) if fancy light meters let you select a type of film and weigh the incident light intensity by the films corresponding sensitivity? I guess that would require multiple cells to get a spectral profile, which is probably not worth the cost and effort in a light meter...

Thank you for the graph -- to clarify, the x-axis units are f-stops corresponding to equivalent f-stops for a given spectral range? so at 570nm the relative f-stop would be ~f/89 if your blues at 440nm are at ~f/9? Is this correct? So then the grayscale background would correspond to a greyscale negative density (low density in the yellows relative to blues for an equivalent blue/yellow intensity)? Guess that demonstrates the importance of yellow filters!

On a separate note, I usually don't use a yellow filter, just a UV filter. In principle, shadows tend to hold more of the blue-scattered light, whereas the more directly illuminated portions hold warmer light (orange/yellow); so not using a yellow filter keeps my shadows a little brighter and decreases the overall contrast? This is typically what I notice, and most of the time I prefer the lower-contrast/softness more than a punchy image. Folks always talk about using a yellow filter to make what you capture 'more like what your eye sees', but I prefer to think about how filtering affects shadow regions and highlights...
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Old November 19th, 2012, 05:29 PM
Bart_van_der_Wolf Bart_van_der_Wolf is offline
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Quote:
Originally Posted by Nick Masson View Post
Thanks for the explanation Bart. and yes, I meant open up not stop down, sorry. I guess not very many people use film anymore, but I wonder (perhaps you know) if fancy light meters let you select a type of film and weigh the incident light intensity by the films corresponding sensitivity? I guess that would require multiple cells to get a spectral profile, which is probably not worth the cost and effort in a light meter...
Hi Nick,

AFAIK they are color 'calibrated' to a standardized neutral response to offset the specific (IR) color sensitivity of their photocells (often silicon photo diodes).

There is surprisingly little information about that, even for a top of the line product like the Sekonic L-758DR. It is possible for the user to calibrate it, but only the tonecurve/dynamic range. Their Color temperature meters do offer a choice between film and digital sensor response, due to the differences in peak R/G/B transmission for film and digital sensors.

A company like Gossen mentions the use of two colour-corrected sbc silicon photo diodes as lightsensor for their DigiSky model.

Quote:
Thank you for the graph -- to clarify, the x-axis units are f-stops corresponding to equivalent f-stops for a given spectral range? so at 570nm the relative f-stop would be ~f/89 if your blues at 440nm are at ~f/9? Is this correct?
The luminance steps in the chart background are calibrated (in the original chart) to 1/3rd stop intervals. Given the chosen pigments for the color bars, their visual brightness impresssion is equal to the marked step in the background. So the 570nm bar is 10/3rd or 3 1/3rd of a stop brighter than the 440nm bar.

Quote:
So then the grayscale background would correspond to a greyscale negative density (low density in the yellows relative to blues for an equivalent blue/yellow intensity)? Guess that demonstrates the importance of yellow filters!
Well, one really should use colored light instead of reflection from a pigment. The chart does allow to get a quick sense of (film) sensitivity to colored light, versus human vision.

Cheers,
Bart
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