Contrast on cold light head

Hi,
I've got a Cathomag Cold light head for my Devere 5x7 enlarger. How do you adjust the contrast? I can't see any knobs or anything. Do I have to make a filter holder to go under the lens? Or just use various grades of paper.Thanks,
Mark

what sort of cold light head? Some have blue and green lamps and some have white light only.

If its blue and green lamps then its the time of each lamp which controls contrast. If its white light only then you'll need some filters.

The blue and green lamp units usually have a controller for contrast I think but you could use filters with them too, if you can work out the right combination of blue and green times that would normally be in white light.

I have white light. So, under the lens filters, or can i stick them somewhere else? Between the neg and the light ?
Thanks,
M

yes you can put them anywhere. between light and diffuser/condensor is best but you can put them under lens if they are optical quality. Ilford ones are. At least when they are new, not if they are dirty or scratched.

If there's a filter drawer you use the larger Multigrade filters, if not the below the lens set and the holder attaches to the lens.

I used a De Vere Cathomag head on a Johnson's V45 enlarge for years (more than 20) with a below the lens set of Ilford Multigrade filters with no problems at all. Only reason I stopped was I gave the enlarger away after acquiring a De Vere 5108 with a colour head.

Ian

The filters are designed for incandescent light. Some cold light "white" light sources have a different spectrum, especially if they were made in the days of graded paper before VC papers became so popular. The light in some cases was somewhat blueish. So the spacing between grades when using filters might be not uniform, and the low contrast filters might not give you low enough contrast. Can you borrow a set of filters to try?

I was doubtful that under the lens filters would not degrade the sharpness of the print, but I have read that there is no degradation as long as the filters are in excellent condition.

Edit: Ian chimed in while I was writing, so his experience is your guide.

If you use filters on the enlarging lens itself, they should be high quality glass ones just like you'd use over a camera lens. All you need is
one deep blue filter and a deep green one, then learn how to "split print". But if the filters are used in a filter drawer above the negative
carrier, you can use various less expensive sheet filters sold for multigrade papers. My own cold light put out blue-green light, so is especially
suited to split printing, though it can be used completely without filtration too; and graded papers see only the blue light anyway.

Thanks

So you use variable contrast papers with only blue and green filters?
M

Variable contrast paper has different emulsions that are sensitive to green and blue light. You might try some additional yellow filtration with the Cathomag when using the Ilford filter set.

modern filtration uses VC paper with Magenta and Yellow filters. The ilford and Kodak filters contain Magenta, Yellow or a mix of the two.

VC paper is sensitive to blue and green light.

Magenta and Yellow are secondary colours and are called subtractive filters becasue they subtract colour from the light. Magenta subtracts its opposite which is green. Yellow subtracts its opposite which is blue.

Adding Magenta filtration increases contrast. So that means taking out green light increases contrast which means green light is the soft contrast component and that blue light is the hard contrast component.

Adding Yellow filtration reduces contrast. Yellow fitration takes out blue light leaving a bias of green light. So again, blue light forms the hard contrast component and green light is the soft contrast component.

Additive filters add their own colour so a blue filter adds blue and a green filter adds green. Only thing is that a blue filter can't add more blue than is already in the light but it does filter out some of the green light too but not as much as a magenta filter can. Same with a green filter. It can't add more green than is already in the light but it does filter out some of the blue light but not as much as a yellow filter can. So subtractive filtration is preferred and subtractive filtration is the norm these days. So you use Magenta and Yellow filters to control print contrast on VC paper.

p.s. In reality its the relative levels of blue and green light which controls the resulting contrast. i.e. both the green and blue components combined which give the final contrast.

RobC said:

p.s. In reality its the relative levels of blue and green light which controls the resulting contrast. i.e. both the green and blue components combined which give the final contrast.

Click to expand...

Exactly... the tradition of using Magenta (Red + Blue) or Yellow (Red + Green) filters for Multigrade printing gives you red light in both cases... and that helps you see so you can focus, etc. The red light doesn't expose the paper at all, just makes the light brigher for seeing.

Bill Burk said:

Exactly... the tradition of using Magenta (Red + Blue) or Yellow (Red + Green) filters for Multigrade printing gives you red light in both cases... and that helps you see so you can focus, etc. The red light doesn't expose the paper at all, just makes the light brigher for seeing.

Click to expand...

Exactly and my additive head has a red light that goes on for focusing.

Ok,thanks.

Or I just punch the blue channel for high contrast, and green for low contrast, or any combination for in between using my additive colorheads. But since these are often in use for actual color printing, I generally just use the blue-green cold light enlarger for black and white work.

Rosco Gels, green and blue. Make a cardboard frame. B&H sells them and they are not expensive. They stand up to heat as the are lighting gels. full green for lowest contrast, full blue for max. Lay on top of neg carrier or in filter slot if you have one.

And by using a set of Ilford Multigrde filters you get the benefit of them being speed matched (same density) on a tone in the print which will be a highlight tone. That means when you change filter the print time remians fairly constant for that highlight tone.

Thank you very much, i shall experiment.

My own cold light requires a yellow color correction filter, I think I have a Kodak YCC80 (?), very light yellow to place directly on the diffuser, or the MG filters will be skewed . . .too much blue. Somebody already mentioned this.

Filters can be placed directly on the negative carrier. The millimeter or so between the emulsion and the filter takes any scratches way out of focus.