Originally Posted by benjiboy
Your meter has silicon cells it should be reasonable through a filter, the early CdS cells are woeful in lots of conditions? Have you tried it?
I'm in the camp of metering the subject directly, then apply the filter factor.
An important point is where you get your filter factors from. I recently posted in a thread about filter factors for some sharp-cutting red filter (like Wratten 25). Someone was using factors supplied by the filter manufacturer, which I maintain is the wrong place to get them from - you should ideally get them from the film maker, and ideally for the light source you use.
Since I went to the trouble of writing an explanation, I'm also going to paste it in here:
Most of the time your exposure won't be so sensitive to filters - this is sort of a worst-case situation. The #25 red cuts sharply at about 600 nm (at 580 nm and lower virtually nothing comes through) so exposure only occurs from there on up. This means that there can be a big exposure difference between films with different amounts of red sensitivity, such as the two listed.
Let me demonstrate why this matters. We now know that your B+W 090 (light red) is equivalent to Wratten 25, right? Looking at two Kodak film data sheets, T-max 100 and the obsolete Tech-pan, here are Kodak's filter factors:
Wratten 25 (red), Daylight: T-max factor = 8 (3 stops), Tech Pan factor = 3 (1.6 stops)
Wratten 25 (red), Tungsten light: T-max factor = 4 (2 stops), Tech Pan factor = 2 (1 stop)
So Kodak, who ought to be the most knowledgeable about their own films and filters, have the filter factor for this one specific filter anywhere from 2X to 8X. In other words, the exposure correction should be somewhere from 1 stop to 3 stops, DEPENDING on the film and light source. THIS IS WHY THE FILTER MAKER CANNOT PUT A (reliable) BLANKET FACTOR ON THIS FILTER. (If your only data is from the filter maker, intial exposure tests are probably worthwhile.)
If anyone is inclined to meter THRU the filter, they ought to consider how the meter would handle the difference between these two films: Tmax 100 vs Tech Pan.
Originally Posted by Mr Bill
I know the spectral sensitivity of Silicon Blue cells is better than Cadmium Sulphide ones Noel, your'e quite right, but they are still not perfect or as good as photo electric meter cells it's just the method I tend to use myself that works for me.
Originally Posted by Xmas
I'd wish Iwould have a better understanding of the spectral sensitivity of film and how that is affected by filtersI have and understand the diagrams but,thry don't seem to correlate to actual testing.for example a sharp-cutting red should block the light of most laser pointersbut,I can still see the pointer through it.How can that be? I don't trust tspectral sensitivity diagrams.
Originally Posted by StoneNYC
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If using one of my Nikon SLR's, I filter thru the filter that is mounted on the camera. Never had a problem. If using any other camera, I use a handheld meter and apply a filter factor. Seems like a lot of work otherwise, removing filter, holding it in front of a meter, trying not to drop anything, replacing filter on lens. Other than landscapes, I would not think it would work anyway. JMHO
How exactly do you mean that it doesn't work? No change in exposure, incorrect or unexpected additional exposure??
Originally Posted by lhalcong
FWIW, I've always added exposure using the filter's published filter factor, plus my own experience with the filter, particular film and lighting – all as acquired by developing and printing my own work. I have never fully trusted metering through the filter with my spot meter because of the (to me) unknown spectral response, even though it does seem the most logical and direct way of measurement, and despite the fact that Hutchings preferred it in the one and only workshop (Azo printing) I have ever taken.
Ralph, if the laser pointer is red (~ 635 or 650 nm perhaps?), then it should pass easily through a Wratten 25, or even the higher cutting #29. Either will transmit over 80% of that light. I think you're just having a temporary brain freeze about filters. If a filter looks red, that's because it lets red light come through.
Originally Posted by RalphLambrecht
If you have the companion tri-color filters to #25, the #58 green or the #47B blue, either of these should stop a red laser dead. (Assuming around either 635 or 650 nm).
However this works out I make one observation:
1. If you use a hand held meter, it seems very impractical to take the filter off the lens just to meter through it. Instead just meter and add the filter factor.
2. If you use an in-camera meter, it seems very impractical to remove the filter to take a light reading and then add the filter factor.
Two main scenarios. I can tell from experience that my Pentax 35mm SLRs do not measure correctly through red filters. The scenes always come out underexposed.
Therefore, when I use filters I always use a hand held meter and add the filter factor, and those negatives always come out right.
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Gee I didn't know that about the light sensors and light meters. So I assume that light meters see the whole visible spectrum?
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