Dave, I have to agree with Patricia. Having used quite a few colour heads on different enlargers with VC papers, including the LPL C7700 and the LPL 7450 there is an exposure difference.
Granted it's tiny, but no matter how careful I am in using the combined Y&M filters, there is a subtle difference. I measure it at about 1/12 of a stop difference in time when I shift from grade 3 1/2 to grade 3 3/4. I could not get it any better than that.
These days I run a DeVere 504 Dichromat and the exposure change is almost identical.
When I change a full grade of paper filtration, say from grade 2 to grade 3, I get about 1/8 of exposure difference. The difference being that I have to add 1/8 of a stop more time. Conversely, when going down I have to reduce time.
I calibrated my colour enlarger head with the current Ilford VC papers that I use, to ascertain the exposure difference. What I found was interesting.
I first did this with my Meopta fitted with a colour head, followed by the LPL 7700 and then the LPL 7450, finally I did it with my current and probably last enlarger, my DeVere 504.
I found that from grade 0 through to about 1 3/4 the exposure time required remained constant, from grade 2 through to grade 4 1/4 there was always an exposure shift required. The exposure shift between noticeable grade changes, doesn't appear to be linear. Things move faster between grade 2 1/4 and grade 3 1/2 then slow down either side. At least that is what is reasonably apparent with my limited testing.
Of course one has to realise that there really isn't any such thing as a set paper grade when you are using dial in filtration, you can have a poofteenth over grade 3 1/2 and find that that is perfect for that particular neg and paper, on that particular darkroom session.
When using a professionally set-up Ilford head, I've found they give near perfect grade change and exposure change seamlessly. In fact as the exposure is counting down, you can hit a different grade of filtration and your highlights will remain the same, perhaps a bit more or less contrastier, but nethertheless, density will be virtually perfect. When working well, I believe they are the best thing in a darkroom. Second best is a colour head or a B&W VC head.
The exposure changes are generally minimal between grades if using a colour head and dialing in both Y&M, but in the four personal enlargers I've had with colour heads, I have always found a slight density difference.
With experience I can guestimate the slight density difference very well.
There is of course the possibility that different manufacturers colour heads will work better and don't require exposure changes between grade changes, that would be interesting to know.
Mick.
