Color Filtration Question

[newcan1]

I have a very basic 35mm enlarger that I plan to take home, that has a filter drawer. I have a full set of Calumet filters that I can use for color printing with this setup. (My principal darkroom is at another location).

In addition to the usual cyan, magenta, and yellow filters, the filter set also includes red, green, and blue.

With my dichroic enlarger, I recently did some color printing where the correct filtration was say 40Y, 50M.

If I want to use the minimum number of my Calumet filters to approximate this setting in the basic enlarger that I am taking home, would I use say 40R, 10M? In other words, does 40Y + 40M = 40R? Or is it 80R? I apologize for my ignorance; it seems intuitive that the answer would be 40R, but I don't have a reference that tells me what the result is of adding say magenta and yellow together, in terms of the amount of red.

Once I have my mind around this, I think it would be relatively easy to use these filters in lieu of a dichroic head. The plus to me is that I can then have a basic darkroom at home that I can use evenings, instead of the one that I have that is not at home.

 

[Leigh B]

The fundamental problem is that you cannot substitute primary filters for an asymmetric subtractive filter pack,
e.g. 30Y + 50M has no equivalent in your RGB set.

You cannot combine (stack) RGB filters. That's the whole reason subtractive filters exist.

- Leigh

 

[newcan1]

I realize that, at least in the case of pure RGB colors. But 30Y plus 50M contains within it a component of red. My red filters are graded in units the same as the C,Y,M ones. Rather than use a bunch of yellow and magenta filters, I figure I could insert a graded red filter for that red component, which in your example is 30Y + 30M, and add magenta to bring the magenta component to the correct level. My assumption is that the correct filter pack would then be 30R + 20M, using your example. Please note that the red filters I have are indeed graded in units, ie they are not pure narrow band filters and represent, I assume, an equal combination of Y+M. My only question was how to do the appropriate calculation so I can use a graded red filter instead of two equal Y and M filters for that component of filtration.

 

[Prof_Pixel]

newcan1,

You are correct.

A Magenta filter passes Red & Blue and modulates Green
A Yellow filter passes Red & Green and modulates Blue
Combing the two will pass Red and modulate Green and Blue - this is a Red filter

So in a perfect. sharp-cutting bandpass filter world, you can replace a 30 M and a 30 Y with a 30 R filter. However, since filters are far from perfect, your milage may vary considerably.

 

[Bill Burk]

The fundamental problem is that you cannot substitute primary filters for an asymmetric subtractive filter pack,
e.g. 30Y + 50M has no equivalent in your RGB set.

You cannot combine (stack) RGB filters. That's the whole reason subtractive filters exist.

- Leigh

Hi Leigh,

Usually you are spot-on but in this case OP has the concept straight. That was the intent of the RGB CC filters.

With the CMY filters you stack two different colors because each subtracts only a third of the spectrum... But when you find you are subtracting the same amount of two colors, then you can replace it with ONE of the RGB filters.

...From Todd-Zakia Photographic Sensitometry...
These rules apply: C + M = B; C + Y = G; M + Y = R; C + M + Y = Neutral. Thus the combination of CC30M and CC30Y gives about the same effect as would a CC30R.

Funny thing is that I always gloss over color chapters in the book since I rarely ever need to apply it. But I read that paragraph just last week so it was fresh in my mind...

 

[Leigh B]

These rules apply: C + M = B; C + Y = G; M + Y = R; C + M + Y = Neutral. Thus the combination of CC30M and CC30Y gives about the same effect as would a CC30R.

Yes, Bill, I'm well aware of how color filters work. I taught the color class at the photography school.

The point I made in post #2 above is that the equivalence only works when you have equal densities of two subtractive primaries.
It fails if they are different, because you can't stack two additive primary filters; nothing will get through the pair.

- Leigh

 

[newcan1]

Thanks all, I am glad I had it right. And Leigh, I do take your point - I can use an R,G or B filter to eliminate two subtractive filters of equal strength, but beyond that, I can't stack RGB filters of different color.

Thanks for the help, all!

 

[Bill Burk]

Yes, Bill, I'm well aware of how color filters work. I taught the color class at the photography school.

The point I made in post #2 above is that the equivalence only works when you have equal densities of two subtractive primaries.
It fails if they are different, because you can't stack two additive primary filters; nothing will get through the pair.

- Leigh

No problem, I knew you knew the stuff...

Maybe it wasn't very common to see the RGB CC filters or maybe because you don't HAVE to use them...

I think they would be more critical in front of the lens when trying to keep down the number of dirty surfaces.

For your example, you can turn

30Y + 50M = 30R + 20M