Neutral density with a colour head

[MattKing]

EXCEPT as I stated before, using only Yellow and Magenta filters CHANGE THE CONTRAST. Therefore using only Yellow and Magenta filters do not have the same effect as closing down the aperture. That is exactly as it works, even though you choose to disagree.

Using equal amounts of the three filter produce Gray which is neutral which Yellow and Magenta is not.

Sirius,
The cyan filter has (essentially) no effect on variable contrast paper, as all it does is attenuate red, which the paper is not sensitive to.
If you make exactly the same change to the magenta and yellow filters, you leave the ratio of blue and green light unchanged, adjusting only the relative amount of red light, thus making no change to the contrast.
The only reason I suggest an adjustment to the cyan filter, is that no filter is 100% accurate.

 

[cowanw]

Sirius, will you agree that the use of filter settings at a grade two level (or 2.5ish) will yield the same contrast as white light will, while requiring more exposure time?

 

[Sirius Glass]

Sirius, will you agree that the use of filter settings at a grade two level (or 2.5ish) will yield the same contrast as white light will, while requiring more exposure time?

Yes

 

[Doremus Scudder]

Sirius,

I think you've got it wrong here. Since the red component of the light is of no significance to the paper, one can safely not worry about attenuating it (i.e., forget about cyan). Equal amounts of yellow and magenta (in CC units) will yield neutral density for all the light the paper sees and provide a brighter image on the easel.

That said, I would find it easier to use a neutral-density filter, a lower-wattage bulb or simply stop down more than fiddling with CC filters. Still, if the OP has CC filters at hand, that would be a quick way to accomplish his goal.

Trying to accomplish the same thing as CC filters with the dichroic filters in a color head will work as well, but may not be as accurate due to a number of factors. However, the result would just be a small change in contrast, which could be easily compensated for by dialing in more or less of one or the other colors.

Best,

Doremus

 

[pentaxuser]

This is becoming a fascinating subject. The Ilford dual filtration for grade 2 is 45Y 24M for some enlargers( Durst 130M group) and gives, as I understand things, the same grade on VC paper as paper which is unfiltered. Does this mean that if I want grade 2 but the time is too short with 45Y and 24M I can simply double both of the dual filtrations ( 90Y and 48M)and then the paper requires double the exposure but the grade remains at 2 because the balance of Y and M has not changed?

If I am using say a grade 2 MG filter and not the dual filtration system for colour heads but need an extra stop of exposure I can add 30M and 30Y from my colour head to gain an extra stop so nothing except exposure time changes and the grade remains at 2?

What I cannot do if I wish to retain the same grade is use dual filtration for the grade and then add 30Y and 30M as this changes the balance between Y and M. I would end up with 45Y and 24 M ( grade 2) plus 30Y and 30M giving 75Y and 54M which is not the same as 90Y and 48M ( 2x both M and Y for grade 2) .

I take it that the difference between Y and M which is 21 (45-24 = 21) is to be ignored otherwise there is an intuitive case ( possibly completely wrong?) for simply adding the difference of 21 to both the filtrations giving 66Y and 45M.

If I am right with doubling both filtrations to get double the exposure time then is there a constant here of 21 ( 45-24) that can be used if I want to increase exposure by 50% instead of 100%? So if I have 5 secs originally but want 7.5 instead of 10 I can add 30.5 ( 21 + half of 21) to each giving 75.5Y and 55M

Can we in the spirit of scientific hypothesis try to "do this whole subject to exhaustion" in terms of its ins and outs

Thanks

pentaxuser

 

[MattKing]

Does this mean that if I want grade 2 but the time is too short with 45Y and 24M I can simply double both of the dual filtrations ( 90Y and 48M)and then the paper requires double the exposure but the grade remains at 2 because the balance of Y and M has not changed?

No - the filter settings are not linear, they are logarithmic.
To maintain the same ratio of blue and green light, add the same number to each setting. 75Y and 54M will give you the same contrast, and most likely one stop less light (depending on which scale is used by your light source).

 

[Sirius Glass]

No - the filter settings are not linear, they are logarithmic.
To maintain the same ratio of blue and green light, add the same number to each setting. 75Y and 54M will give you the same contrast, and most likely one stop less light (depending on which scale is used by your light source).

So much simpler mathematically to use 30C + 30Y + 30M and plus the Y and M for the desired grade and be done with it.

 

[koraks]

No - the filter settings are not linear, they are logarithmic.

I'd expect them to be exponential, wouldn't you agree? The question (for me) is: at what rate. Power of 2? Power of 4? Power of 10? Something else? I've not yet been able to find good information on this. Does anyone have a suggestion on how to figure this out?

Edit: my logic may be more based on an additive system; the fundamental question still stands though.

 

[Mr Bill]

I'd expect them to be exponential, wouldn't you agree? The question (for me) is: at what rate. Power of 2? Power of 4? Power of 10? Something else? I've not yet been able to find good information on this. Does anyone have a suggestion on how to figure this out?

Edit: my logic may be more based on an additive system; the fundamental question still stands though.

The cc filters are essentially rated in optical density, but with a decimal point shifted over. So a 30 cc gelatin filter will have a measurable density of about 0.30. Density is expressed as a base 10 logarithm, so the 30cc filter attenuates in proportion, actually inverse proportion, to 10^0.30.

More plainly the 30cc filter should transmit 1/(10^0.30) = 0.50, or 50% of the light. If you were to stack up two such filters - a 30cc plus 30cc, it would transmit 0.50 x 0.50 = 0.25, or 25% of the light. When you stack filters, the cc values are summed, so the 30cc + 30cc are equivalent to 60cc. Working the math gives the same result as the previous, 25% transmission: 1/(10^0.60) = 0.25.

I should point out that the color filters follow the rule only for the "color" they attenuate. For example, the cyan filter being discussed only attenuates "red" light; the "green" and "blue" light are essentially unaffected (actually a slight amount by virtue of the surfaces being added). Anyway, if you were to read cyan filter with a color densitometer designed for reading color neg film, you would expect the R value to be roughly equal to the cc value (with decimal point shifted), whereas the G and B values should be very small. It is, in fact, essentially the same thing as reading the amount of cyan dye in the film. The densitometer doesn't know if it is reading cyan dye in the film or a cyan filter - all it knows is how much the "red" light has been reduced, and it displays this as a density value.

 

[koraks]

Mr Bill, thank you very much. That was insanely useful, and also touches upon things I've been reading.
Given the different maximum cc values on different brands/types of heads (think of Durst maximum 130 VS maximum 170), I wonder if the differences in fact are differences in minimum transmission (or maximum opacity) or if the relationship between cc''s and transmission is in practice different than the 30=0.30 relationship you mention. Do you happen to know this as well?

 

[MattKing]

Does anyone know if the settings on all the different types of colour heads are actually calibrated to the same cc units?

 

[Mr Bill]

... or if the relationship between cc''s and transmission is in practice different than the 30=0.30 relationship you mention. Do you happen to know this as well?

You're welcome.

I really don't know first-hand; my industry experience has been mostly with either the physical CC or CP filters (which is what I was describing) OR the systems using so-called filter paddles. (The "paddles" are hard-cutting dichroic filters - essentially all or nothing - which are flipped into the light path to terminate a color's exposure. So they are controlled by time, not density. The user interface was typically set up to show equivalent CC units, but since it was controlled by software the units could be anything one wanted.) But a little more info follows.

Does anyone know if the settings on all the different types of colour heads are actually calibrated to the same cc units?

I found an online manual for the Durst Laborator 1200, a pro-grade enlarger with colorhead available, that has something to say on the matter. Their CLS 501 colour head has "Durst densities" from 0 to 130. Here's a quote:

If you are used to colour heads calibrated in CC or CP filter values, you will notice that the filters of the CLS 501 colour head have considerably higher filter densities at equivalent scale settings.
Kodak filter values need to be 1.5 times as high as equivalent Durst values for the same effect.
CIBA filter values need to be 1.5 times as high as equivalent Durst values for the same effect.
Agfa filter values need to be twice as high as equivalent Durst values for the same effect.

So it appears that there have been at least a few different scales.

 

[MattKing]

So it appears that there have been at least a few different scales.

Thanks Mr. Bill,
I'll try to keep this in mind whenever the "30 steps = one stop" idea is part of the discussiion.

 

[Bob Carnie]

Thanks Mr. Bill,
I'll try to keep this in mind whenever the "30 steps = one stop" idea is part of the discussiion.

30 magenta is 1 stop difference but not 30 yellow... much less

 

[koraks]

So it appears that there have been at least a few different scales.

I suspected this indeed - although I've never been in a position to verify it. Thanks once again! I may do some measurements and experiments just to get a feeling for this.
As to the base-10 logarithm - this actually confirms what I guessed on the basis of the paper curves for Kodak Endura, which show a base-10 exponential pattern as well. But when experimenting with a light source which I attenuated in the same exponential way, my visual (non-measured) perception was/is that it is way too steep and a base-2/squared output curve is much closer to the visual impression I get when looking at the output of my durst color head. Very subjective of course, and it does warrant some actual measurements for which I'll have to rig up a test setup. But it'll remain some guesswork since a good/reliable color-sensitive measurement is pretty much beyond my practical limits.

 

[koraks]

30 magenta is 1 stop difference but not 30 yellow... much less

That would suggest that the yellow response of the paper is different - which isn't the case. Look at the Endura curves; they track perfectly in the linear region. Probably the same for Crystal Archive, but I'd have to check.

 

[markbau]

Did some printing yesterday and dialled in 100 of C,M,K. I didn't get the step tablet out but it seemed to knock 1/2 a grade off contrast and 3 stops off the exposure. It achieved what I wanted, to extend exposure time when printing small prints.

 

[Sirius Glass]

Did some printing yesterday and dialled in 100 of C,M,K. I didn't get the step tablet out but it seemed to knock 1/2 a grade off contrast and 3 stops off the exposure. It achieved what I wanted, to extend exposure time when printing small prints.

Good I am glad all this discussion led to something useful for you.

 

[Pieter12]

I have to make some postcard size prints and I'm finding the exposure times to be ridiculously short, less than 5 seconds which isn't enough for burning and dodging. I can't stop my lens down any more due to diffraction. I'm using Ilford below lens filters. If I added equal amounts of Y, M and C that would extend my exposure times but does anyone see any problems with this?

I would use the color head instead of the below-the-lens VC filters, according to this chart: https://www.ilfordphoto.com/wp/wp-content/uploads/2017/03/Contrast-control-for-Ilford-Multigrade.pdf
Then use an ND filter on the lens.

 

[John51]

Did some printing yesterday and dialled in 100 of C,M,K. I didn't get the step tablet out but it seemed to knock 1/2 a grade off contrast and 3 stops off the exposure. It achieved what I wanted, to extend exposure time when printing small prints.

Adding equal parts of CMK to tweak the contrast? Sounds good.

 

[markbau]

Adding equal parts of CMK to tweak the contrast? Sounds good.

Please read the title of this thread and my original post. I am using CMY to add density to extend print times, Contrast is controlled by under the lens Ilford filters.

 

[pentaxuser]

Please read the title of this thread and my original post. I am using CMY to add density to extend print times, Contrast is controlled by under the lens Ilford filters.

That was my impression also but you mention K in a previous quote as in CMK and not CMY, hence the confusion maybe. It sounds like K is a typo, was it?

Thanks

pentaxuser

 

[Mick Fagan]

Did some printing yesterday and dialled in 100 of C,M,K. I didn't get the step tablet out but it seemed to knock 1/2 a grade off contrast and 3 stops off the exposure. It achieved what I wanted, to extend exposure time when printing small prints.

Great news.

Mick.

 

[markbau]

Please read the title of this thread and my original post. I am using CMY to add density to extend print times, Contrast is controlled by under the lens Ilford filters.

Yes, it was a typo. The only "K" filter I've ever seen in a darkroom is more commonly known as a dodging tool

 

[John51]

Please read the title of this thread and my original post. I am using CMY to add density to extend print times, Contrast is controlled by under the lens Ilford filters.

I did read the title and the post. I read well enough to copy the CMK typo.

Adding nd altered the contrast. Although that was not your intention, I found it interesting. Being able to drop half a grade might be useful to me when using my condensor enlarger. If nd has a similar effect on contrast with RA4, that could come in handy too.