I am using a Rodenstock Rodagon 135mm on my Omega enlarger to make prints on 16 x 20 paper from 4 x 5 negs. I have had some dust marks on prints recently and after cleaning the negs and filters finally noticed that the dust is always in the same pattern over and over. Ok so it must be the lens. After cleaning it several times I still have the dust issue. I can only assume that the offending dust must be inside the elements that I cannot take apart.

Two questions, 1. where can I send this lens to be overhauled, and 2. Is it worth it for this lens. This is one of the lenses that came with the enlarger, so I don't really know if it is a decent lens, it is just the one I have always used. Maybe there is a really fantastic lens that I just don't know about?

Hi, I think it is dust on the condensers rather than the lens?

Jon

I forgot to mention that I have changed out the condenser to a cold light head. I thought that I would never have problems like this again!!

I can't imagine dust in a lens showing up as a pattern. A degrading of an image, perhaps if the lens is really cruddy; but apparent dust marks on prints in the same place just doesn't compute. I would, also, suspect something else. Do these marks show with different batches of paper?
And yes, the Rodagons are excellent lenses and worth having cleaned, if necessary.

could be scratches/chips on glass neg carrier. I don't know cold light heads but if it has a diffusion sheet above the neg it could be scratches/chips on diffusion sheet. I would be surprised if it were dust in lense. Most lenses have some small amount of dust in them and it makes no difference because one point on the print comes from a large area of the lense which makes the dust spec a tiny fraction of the image forming light(or not) at that point.

The 135 is reputed to be a good lens.

Dust on the lens is out of focus. Can't be sharp, you wouldn't even see it. If your cold light diffusion sheet has dust on it, that will show up assuming that your lens is stopped down pretty far. You can test that by making a print at a wider ap. Use a neutral density filter if necessary. If that is the problem the spots may still be visible but distinctly unfocused.

Otherwise, there would have to be something in between either the light source and film, or between the film and the lens (in either case, close to the film) or between the lens and the paper (and close to the paper). Or, embedded in the film (negative). Or, part of the picture.

It it has gas, air and spark, it ought to start, right?

I can't tell if the dust is in the lens or elsewhere, but my Rodagon 150 has the front group screwed onto the mount/stop assembly, so you may be able to clean 2 more lens surface....
Of course, you might be extremelly carefull here !

Thanks all,

I don't use a glass neg holder, so that is eliminated. I will closely examine the diffusion on the front of the head maybe that is it. And I will try a new batch of paper. I do use varycon and they are known to have inconsistencies in their paper. Thanks for reinforcing my thoughts on the lens. I just couldn't figure out what else it could be. I print around f11 to f16 on the lense and did take it apart and could only see minor amounts of dust in there. I have been using this lens for years and never had this problem.

Appreciate all you responses.

If it is something in the paper (which seems doubtful) it should not be the same on consecutive prints. It is possible, however unlikely, that in coating there could be some defect that repeats itself at intervals conforming to the size of the paper you are using. The test for this would be to turn a sheet of paper around 180º and make another print. If the pattern of spots also turns, it would be in the paper, but if not it would be in the enlarging system. Most likely, seems to me it is stuff on the diffusion material in the head that is right above the negative surface.

It is not the lens. Do N O T start disassembling your lens to fix this. Are you pre-flashing any of these prints ? If so, how ?

i vote for the diffuser. or on the neg (you might have missed it)

You can also turn the lens 180 deg to see if that changes anything.

Could it be glass elements are decemented...

Not to rain on your parades - but the last two suggestions I wouldn't take too seriously (not sure if you were joking-?) - but anything to do with the lens should not be considered. It is physically impossible to form a sharp image of anything in or even NEAR the lens assembly. It MUST be within the DOF of the negative plane OR the print.

Is your cold light a round one that you can rotate by hand? I had such a cold light once and, when I rotated it while looking at the white light projected on the baseboard, I was amazed at the unevenness of illumination as well as dark spots that moved as I rotated the cold light.

Gary,

East Snook, TX

Thanks for the advice. It was dirt on the diffuser, just couldn't see it until it turned it on.