This post is repeated here in chances that some may not see it in unmoderated Photonet.

While further checking out my newly setup Bessler 4x5 MCRX as concerning even illumination, (Dichro head), I've found as much as 9/10's of a stop difference between the center of the projection and the corner of a print, that would be about 8x8 as projected on a white peice of paper using a 6x6 neg carrier.

A previous response to another question educated me as concerning light falloff at distances, and that falloff was normal. (1/2 stop was alluded to on a 8x10.)

My thought as a possible way to even this out would be to create a density mask and use in it the filter drawer which is just above the lens, or in the color filter drawer just beneath my Dichro head. I would expose a peice of film at the size of the filter needed.

My question is 1. Is this is possible? and 2. If not, what can be done? 3. If possible how do I figure exposure to get the proper density in the filter negative and how do I do it on my enlarger.

Basically I don't see why this uneven illumination needs to put the chore of burning and dodging on me, out side of any manipulation that I would want to do.

I would like to find out how others may have dealt with this, and what enlargers others have found to have more even illumination.

With the extra manipulation currently involved, I may as well scan the negs and use Photoshop. Sure seems a better, faster, and sure way to get the results I want without spending hours in the darkroom. I appreciate your insights.

-- Wayne Crider (waynec@apt.net), March 30, 2001

Answers

Before you fix the problem, how even are your camera lenses and how do your images look? If you have some falloff with certain lenses, a little enlarger falloff might not be such a bad thing, at least for images made with those lenses.

-- David Goldfarb (dgoldfarb@barnard.edu), March 30, 2001.

Good point David, but I use pro lenses in my 35mm and MF shooting as well as the older lenses which might have problems with my 4x5. At one point I would need the mask, if useable. Besides, the illumination was not consistent corner to corner or side to side. I'd would also like it closer than 1/2 stop. But your point makes me think that I will have to pay more attention to my older lenses. Previously they were used for color only, and never drew my attention as there being a problem.

-- Wayne Crider (waynec@apt.net), March 30, 2001.

Wayne, Maybe you're dealing with some other variables: the effect of enlarging lens, as all of them suffer from some sort of fall-off, and the way you're measuring this. A blank sheet, printed for medium gray, using different f-stops and different lenses may show clearly how deeply in trouble you are. Good luck! Cesar B.

-- Cesar Barreto (cesarb@infolink.com.br), March 30, 2001.

Wayne:

That much of a fall off in the corners seens awfully high. I have a Beseler 45MXII with a Dichro 45S head. I've never measured the fall off on mine, but, if I had that much fall off, it would be rather obvious when printing. I would imagine that something is wrong. You say that there is a filter drawer just below the color head. On mine there is no filter drawer in that location, the dichroic head attaches directly onto the plate atop the upper bellows. The upper bellows is then fully compressed until the bottom of the head is nearly in contact with the negative carrier. Maybe there has been a change in design over the years, but maybe you should check out that possibility.

-- Ken Burns (kenburns@twave.net), March 30, 2001.

Ken, that's pretty much what my reaction to this question was as well. We have 2 Dichro 45s heads here, and I've never checked them with a meter or anything, but they're right on. I, too, think I'd notice it in our prints, since we shoot alot of tabletop on gray seamless. The only thing I can think of in addition to what Ken has said, is to check the diffusion chamber inside the head to make sure you have the right one, or to see if it's seated properly.

-- DK Thompson (kthompson@moh.dcr.state.nc.us), March 30, 2001.

Wayne:

One other thing I thought of after submitting my post. Are you using the condenser chamber unit with your color head? If so, is it properly focussed?

-- Ken Burns (kenburns@twave.net), March 30, 2001.

No, I'm not using the condenser chamber, although I do have one for my condenser head. I was more interested in using the Dichro head. Everything with the head appears fine; Two lamps both burning and difussion glass fine. Nothing appears whacked out of alignment.

-- Wayne Crider (waynec@apt.net), March 31, 2001.

Lenses, especially wider angle lenses, likewise produce light fall off at the edges unless corrected by a center filter. The enlarger light fall off is the reverse of the lens light fall off and so tends to correct the lens light fall off. In other words, enlarger light fall off within reason isn't necessarily a bad thing since it tends to correct light fall off in the negative. Apart from that, what you are talking about doing is basically making a center filter for your enlarger, something analogous to the center filters made for wide angle lenses. I would think it would be virtually impossible to create such a filter that would work properly with all of your negatives (assuming you use more than one camera lens) and at all of your different enlarger head heights since the enlarger light fall off presumably varies with the height of your enlarger head.

-- Brian Ellis (bellis60@earthlink.net), April 01, 2001.

Wayne, though I never done it, I can't see any reason why a density mask in the flter drawer shouldn't work. Expose a round dark object on a white background as very unsharp on some clear film. If you make it to a density of 0.3D it should darken about one f-stop. I think the grade of density is not so critical, unless there's not too much. So I would make a very, very pale gray film mask, and try with it. Keep the transmission really subtly, otherwise the density area will vary with the f stop used in enlarger lens. I think this is a brilliant idea! Jan

-- Jan Eerala (jan.eerala@itameri.net), April 01, 2001.

I think the most acurate method of customising your centre spot filter would be to place a clean piece of paper into your easel, illuminate it with your enlarger, and photograph the paper with a camera which produces negs in the size of the filter holder. Use the resulting neg as your filter. This should give a proportional fall off on the filter that matches the fall off on the the enlarger. You will probably have to experiment with neg density to get it right.

Of course, you will have to produce a filter for each format film you use in your enlarger.

Graeme

-- Graeme Hird (goldeneyephoto@hotmail.com), April 01, 2001.