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Going back in time, films used to respond to a more limited range of the visible light spectrum. These orthochromatic films, unlike today's more common panchromatic films, were very sensitive to blue light, had accurate tonal representation of yellow and green light, but were hardly sensitive to orange. The films could be exposed to red light, barely (or not, rather) sensitising the film's emulsion.
The latter is also why darkrooms to this day use red lights, as with black-and-white printing orthochromatic photographic paper is used.

Panchromatic film, which today has mostly displaced orthochromatic films, does respond to higher (red) wavelengths and some films even cover the infrared spectrum.

Seeing as how the orthochromatic films responded mostly to blue light, would using a blue filter on modern panchromatic film replicate the tonal representation of ortho film? Using a blue filter would deny (most) of red light to pass through, onto the film.
I could not find the answer to this question online, but I have my doubts at my own theory. Mainly, using a blue filter would also mean a decreased sensitivity to yellow/green. As I have a limit knowledge on what filters are exactly out there, perhaps there is a filter (combination) that would come close, if not perfectly replicate this?

  • Didn't orthochromatic film also have a decreased sensitivity to green/yellow compared to the sensitivity of panchromatic film? That is, if the response to blue light is the same for a particular orthochromatic film and a particular panchromatic film, then the orthochromatic film would have a lower response to green/yellow than the panchromatic film would. – Michael C Jul 30 at 9:21
  • @MichaelC good question, to which I do not know the answer. I'm only aware of the differences at longer wavelengths – timvrhn Jul 30 at 9:25
  • Orthochromatic refers to the natural sensitivity of silver halide emulsions (technically colloidal dispersions) which after ripening are blue sensitive unless dyes are added to increase the spectral response to longer wavelengths. – Stan Jul 30 at 18:06
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You can simulate the spectral response with filters. To cut red, use a cyan or blue-green filter. To select for blue or green, use a blue or green filter. If you can find the response curve of the films you're interested in, you can look for corresponding filters. Otherwise, you should shoot some test rolls to compare filters against.

You can see density curves for Kodak filters. Some recommend trying a Wratten 44A filter. (Light blue-green. Minus-red filter.)

W2-44A

For comparison, here's the curve of the Kodak 38A filter. (Blue. Absorbs red, some UV, and green light.)

W2-38A

You cannot use filters to emulate other aspects of the film, such as fine grain with high contrast. Since filters cut light, you may see increased grain. You could try pushing to increase contrast (at the cost of increased grain) or pulling to decrease grain (at the cost of decreased contrast).

  • Good call on using a cyan filter, instead of a blue filter. I still wonder whether these filters won't partially block out yellow/orange light, which it should not – timvrhn Jul 30 at 12:58
  • @timvrhn Added density curves for Kodak 38A (blue) and 44A (blue-green) filters. How to interpret is another problem. – xiota Jul 30 at 13:42
  • Very helpful charts. Together with this useful tool one can get an idea of the range of light passing through the filter. The 44A seems to hit the spot – timvrhn Jul 30 at 13:58
  • @timvrhn Yes they'll partially block out yellow/orange light but ortho isn't sensitive to those wavelengths anyways so it doesn't matter. You can develop ortho under yellow (bright high-pressure sodium vapour) light. – Stan Jul 30 at 18:02
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    You can use the curves to show sensitivity by turning over the graphs (top for bottom) effectively changing density to transmittance. – Stan Jul 30 at 18:12
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I think a blue filter would come close in terms of killing reds. I've used orthochromatic film for LF, which I think you can still get on occasion: the one I used was branded Rollei but I think was actually Adox, who in turn are related to Agfa and Efke in ways I don't understand. For those recent films what a blue filter would not do is give you the kind of very high contrast that they had, but I don't know if historical orthochromatic films had that property.

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Orthochromatic film is sensitive to blue and green light thus insensitive to red light. To replicate using panchromatic film, you must somehow block red light. A cyan filter is a red blocker. Using a cyan (blue + green) does this trick.

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