Single v dual filter settings

[David Lingham]

Is there a discernible difference in tonality between the single and dual filter settings for a dichromatic colour head when printing on multigrade paper? I have been using a dichro head on my De Vere 203 with VC paper for well over 25 years and have always used the Kodak single filter numbers appropriate to the De Vere. Recently I had to make a series of small prints (5x7) and found the exposure time from the single numbers were too short for any print manipulation. In order to create a longer exposure time for dodging and burning I switched to the dual filter numbers. I subsequently noticed a very slight improved smoother tonal response at the same grade settings, to my eye the difference is only very marginal and would probably be lost if I tried to upload any examples. My theory, and please prove me wrong, is that whereas the single numbers use either the hard or the soft contrast emulsion layer in the paper, the dual numbers use both. For example, grade 3 in single numbers is 25 magenta, grade 3 in the dual figures is 23 yellow 56 magenta.

 

[koraks]

No, there won't be a difference. But using multiple filters you can end up with a slightly different contrast grade due to imperfections in the filers themselves and/or the translation tables used.

dichromatic colour head

'Dichroic'. Dichromatic would be something else. The filters are dichroic, which means they're basically interference filters that work by the selective reflection of certain wavelengths in a very thin metal layer that's deposited on the glass of the filters. This sets them apart from e.g. gelatin filters that use dyes for selective absorption. Dichroic filters are more durable and generally better resistant to heat. Sorry for the nitpicking.

 

[Carnie Bob]

What you are doing kind of makes sense, when using a dichroic enlarger for colour and we wanted longer times we would use three filters by incorporating the cyan filter as well. For Black and white I think you are achieving the same thing with your method, as far as a smoother transition of tones with two filters I never heard of this but maybe you have noticed something of value.

 

[RalphLambrecht]

Is there a discernible difference in tonality between the single and dual filter settings for a dichromatic colour head when printing on multigrade paper? I have been using a dichro head on my De Vere 203 with VC paper for well over 25 years and have always used the Kodak single filter numbers appropriate to the De Vere. Recently I had to make a series of small prints (5x7) and found the exposure time from the single numbers were too short for any print manipulation. In order to create a longer exposure time for dodging and burning I switched to the dual filter numbers. I subsequently noticed a very slight improved smoother tonal response at the same grade settings, to my eye the difference is only very marginal and would probably be lost if I tried to upload any examples. My theory, and please prove me wrong, is that whereas the single numbers use either the hard or the soft contrast emulsion layer in the paper, the dual numbers use both. For example, grade 3 in single numbers is 25 magenta, grade 3 in the dual figures is 23 yellow 56 magenta.

seems logical. However, the way I understand it, there are no different contrast layers in Ilford's VC papers but a single layer containing blue and yellow sensitive components with different contrast behaviour, which would call for no difference between single or dual filtration.

 

[koraks]

there are no different contrast layers in Ilford's VC papers but a single layer containing blue and yellow sensitive components with different contrast behaviour

I've been corrected on this a few times, so I've stored it in my memory - this is not the case. It is indeed a single emulsion, but in the case of Ilford's Multigrade paper, there are 3 light-sensitive components, each of the same contrast, but they vary in the degree to which they are sensitive to green light. All three components are sensitive to blue light.

MULTIGRADE papers are coated with an emulsion which is a mixture of three separate emulsions. Each emulsion is a basic blue sensitive emulsion to which is added different amounts of green sensitising dye. Thus, part of the mixed emulsion is sensitive mainly to blue light, part to blue light with some sensitivity to green light and part to both blue and green light. All parts of the emulsion have the same contrast. They also all have the same speed to blue light, but naturally, the part of the emulsion with only a small amount of green sensitising dye has a low speed (that is, is less sensitive) to green light. When the paper is exposed to blue light, all parts of the emulsion react and contribute equally to the final image. This image is of high contrast because of the additive effect produced by three emulsions with the same speed and contrast. The resultant curve has a narrow exposure range and is thus of high contrast.


When the paper is exposed to green light, only the parts of the emulsion with the larger amounts of green sensitising dye react initially. This is because the three emulsions have very different sensitivities to green light. This image is of low contrast because of the additive effect produced by three emulsions with different speeds to green light, but with the same inherent contrast. The resultant curve has a very much wider exposure range and is thus of low contrast. By varying the proportion of blue to green light, a contrast range between these two extremes can be obtained.

The Science Behind Ilford Multigrade

rps.org

 

[bernard_L]

Is there a discernible difference in tonality between the single and dual filter settings for a dichromatic colour head when printing on multigrade paper?

in the case of Ilford's Multigrade paper, there are 3 light-sensitive components, each of the same contrast, but they vary in the degree to which they are sensitive to green light. All three components are sensitive to blue light.

I've asked myself a similar question for a long time, in the specific case of the Ilford MG papers., An optimum D/logE curve is obtained by the relative shifting of the three emulsions (in a single layer). As the "hardness" is modified the D/logE curve of the intermediate (green/blue) emulsion should move towards/away the D/logE curve of the blue-only emulsion at half the rate of the green-only emulsion. Once general exposure is accounted for, this requires two degrees of freedom, that must be properly coordinated to achieve a proper curve at the intermediate grades.

Now, with a dichroic color head, or a blue/green LED head, we have (again, once general exposure is accounted for) just one degree of freedom. Maybe the resulting shifts of the three D/logE curves will be near optimum way, maybe not. One might imagine that the Ilford MG filters are carefully engineered in that respect.

In practice, nobody has seen a demonstrable difference. And then there are Foma papers: two or three emulsions?