Focussing Problems with VC papers

[brian steinberger]

I just started reading Ctein's book "Post Exposure." And in it he talks about focussing problems with VC papers due to the mulitple emulsions being sensitive to different wave lengths of light. I'm wonder how many of you, if any have expereinced unsharp prints when printing with VC papers, or if anyone has even heard of this. Thanks

Brian

[Doremus Scudder]

Brian,

This focusing problem Ctein mentions is a characteristic of some enlarging lenses. If the lens is not corrected to focus near-UV (dark blue) light, to which the paper's hard component is sensitive to, at the same plane as green light, for which the paper's soft component is designed, then you can get a "dual focus" similar to color fringing (chromatic aberration) that results in unsharp prints. I use graded papers, but I imagine you could test for this by making a print using different filtration. Step 1, choose a fairly contrasty neg and set your enlarger for a fairly large print. Focus as carefully as you can using white light. This will be the "green focus" since your eyes cannot see the near-UV which might be focusing on a different plane. Step 2, make a print using the softest filter you have. This should effectively cut out the blue light, giving you a print focused "as you saw it." Step 3, without changing anything except the filtration, make a print using the hardest filter you have. This should cut out the green light and give you a print with blue and near-UV. If your lens has a problem, this print should be less sharp than the first. If there is no difference in sharpness, your lens is well-corrected and you have no problem. Keep in mind that the contrast difference may hamper your evaluation attempts. Try to find a mid-tone area with lots of local contrast that is similar in both prints.

If you do have a problem, you will have to research about lenses, transmission spectra and where things focus, and then find one that does not have the problem. Maybe some VC printers can chime in here. I would imagine that most modern high-quality lenses would be well-corrected for this, but really am not sure.

Best,

[windpointphoto]

Are you experiencing these problems? If not, why care? This sounds like reading too many books.

[Bill_1856]

I definitely experienced this problem! The definitive cure is to return to graded papers. Unfortunately, my favorite graded papers have disappeared from the marketplace.

[John_4185]

Ctein's claim is one of those I have to see to believe.

Bill, got an example? What lens were y0u using? What filters?

[Paul Butzi]

I once tried to replicate Ctein's results in his tests regarding focus and VC papers.

I failed to detect ANY focus shift, let alone any focus shift of the magnitude he described.

My conclusion - I don't know what Ctein was doing, but his results didn't apply in my darkroom.

[neil poulsen]

I've heard of this. Supposedly, Leica auto-focus enlargers had sharper focus than could be obtained visually, because they took UV into account.

Does Ctein say what to do about this? Which lenses, or how new must they be for proper focus? Is it possible to place a uv filter in the light path, say above the diffuser, to correct for this? Some people place about a 40 yellow in some diffusion heads to correct for the abundance of blue. Does this correct the problem?

I could see people having this problem but not being aware of it until it's corrected, or until they do the side-by-side comparison that Doremus suggests.

[Paul Fitzgerald]

Hi there,

Brian, it sounds like a case of excess precision BUT not a fairytale. The old Kodak autofocus enlargers were focused to the paper not to visual focus. All papers use the UV end of the scale to expose, VC. can see the green light, graded paper doesn't.

You can check your enlarger for a problem easily. Put 2 layers of 1/4" foamcore under your easel and critically focus as normal and lock down all adjustments. Now add 2 more layers of foamcore under the easel and expose, remove a layer and expose, ect, ect,. Mark the prints so they don't get confused and check them when dry. You should have 5 prints, #3 should be the sharpest, if not then you have a problem. Any enlarger lens since WWII should not have any problem unless it was dropped. Are you using cold-light or condensers, the condensers could be out of alignment or mis-matched to the lens and make a problem with this.

Have fun with it.

[John_4185]

This seems to imply that people using multiple filters (splitting) will have one image way out of focus. And Paul, 1/4" is a HUGE difference. If this were the case, I would think that enlarging lens makers would have dealt with the issue a long time ago.

[Paul Fitzgerald]

Hi there,

jj, you're right, sorry for the typo, it should have been 1/8". Either way it's a NON-problem. The coating layers on the paper are seperated by how much? .001"? Like I said "excess precision". There is more of a problem with personal eyesight thru the grain focuser, changing filters or filter settings, a truck driving by, ect., than from UV light difference. If the manufacturers had not fixed this, no one could make color prints, they would be really, really, really BLUE and fuzzy.

Now everyone can go over to APUG and see if they are #200,000 poster.

Have fun