I have recently bought a Durst M805 enlarger with colour diffuser head, which I generally use with 6cmx7cm negs. At this stage, I have not yet bought the lens I want (a Rodenstock-APO 105mm), but am testing it using the 80mm EL-Nikkor it came with.

I've noticed a very irritating thing. The image will focus sharply in the centre, and it will focus sharply into the corners, but not both at once. At first I thought it was the lens being too short, but the fact that it is capable of being sharp (albeit not simultaneously with the centre) seemed to discount this theory - though I'm open to being corrected about this.

The enlarger does have perspective control, so my next idea was that the board to which the lens is attached (which can be rotated in every plane) might not be parallel to the baseboard. I have checked this, and as far as I can establish, it is parallel - though I've not got any scientific tools to check this.

Any ideas, theories or tips? Is it the lens being too short? Could it be that I need to invest in some kit to check that the lens is parallel to the baseboard? Any other thoughts?

Many thanks in advance!

-- Ed Hurst (BullMoo@hotmail.com), May 15, 2001

Answers

That is odd. If the center is sharp but your four corners are unsharp by different amounts (a different amount of focus to bring each corneer into focus), then it sounds like your enlarger is out of alignment. But if all four corners are equally out of alignment, could it be possible that your lens has some curvature of field? Almost all lenses have some but stopping down a bit should help - however, excessive stopping down is best avoided for a number of reasons - long exposure times, reciprocity of paper, diffraction limitations etc. The El Nikkors are very good but sometimes, a particular lens may be off due to decentering/manufacturing issues etc. Good luck, DJ.

-- N Dhananjay (ndhanu@umich.edu), May 15, 2001.

If you go ahead and get the 105mm lens you will probably find it has a flatter field, though I must confess I often use an 80mm for my 6x7 negatives with no problem. So it also wouldn't hurt to check your enlarger alignment. There are several tools made for this purpose. You can make one yourself with two mirrors. One needs to have a hole drilled in it--a glass company can do this for you. You put the one with the hole, mirror down, in the enlarger where the negative carrier goes, and the other on the baseboard. All the images of the light source will line up perfectly when the enlarger is properly aligned.

-- Ed Buffaloe (edb@unblinkingeye.com), May 15, 2001.

why don't you send an e-mail to Nikon about this. I would be curious about their response.

-- (ricardo_spanks@yahoo.com), May 15, 2001.

Are you using a glass carrier? Could be the negative is not flat.

-- Chris Ellinger (chris@ellingerphoto.com), May 15, 2001.

I experience the same thing with my EL Nikkor 50mm and LPL 66 enlarger. I suspect the negative is not perfectly flat but I don't have a glass negative carrier to test the difference. Stopping the lens down to f8 compensates for this and I come up with acceptably sharp images.

-- jorge andrada (glamour@mozcom.com), May 15, 2001.

Hmmm. Interesting! Do you focus the centre of the image sharply and allow the depth of field to take care of the rest, or do you try and focus in between to take advantage of hyperfocal range?

-- Ed Hurst (BullMoo@hotmail.com), May 15, 2001.

Are you talking about extreme differences in sharpness between the center and the corners, or slight? Do you notice the difference stopped all the way down? Is the difference equal in all four corners at the same time? If so I doubt the problem is alignment.

If you were using a condensor type enlarger one might think that you had the condensor lenses in the wrong way. But with the diffusion type head that isn't an issue. Could your lens be defective? Lens elements sometimes seperate over time.

-- Paul Swenson (paulphoto@humboldt1.com), May 15, 2001.

This becomes more of a problem with increasing size of roll film. I run into this problem ocassionally with 6x7 negatives and not very much with 6x6 and almost never with 35mm. I have improvised a glass carrier to use when a 6x6 or 6x7 negative refuses to lay flat. I only use the glass carrier if I have a serious enough problem to justify the additional glass cleaning and print spotting. The 80mm El-Nikkor is a great lens and I'd work with it for a while before making an additional purchase. I get great 16x20's with it, sharp grain to the corners. The 105 will reduce your maximum print size over the 80 unless you resort to wall or floor projection or construct an enlarging table.

-- Gene Crumpler (nikonguy@att.net), May 15, 2001.

Ed, this sounds like a case of negative curvature, the best way to eliminate it is by using a glass neg. carrier. Usually the larger the negative surface area the greater the problem that's why 35mm negs. are usually ok in glassless neg. carriers. But by the time you get to 6x7cm and with modern thin film emulsions the neg. is never really critically flat in a glassless carrier. Regards,

-- Trevor Crone (trevor.crone@uk.dreamcast.com), May 15, 2001.

To answer Ed's question, I focus the image initially at the center using a grain focuser then shift the aperture to f8 and then refine the focusing by finding the best overall sharpness overall with my eyes.

This of course makes it difficult because at f8 the image is already quite dark. This also practically makes the grain focuser next to useless. I wish I didn't have this problem in the first place.

-- jorge andrada (glamour@mozcom.com), May 16, 2001.

Try taping a negative into your carrier, stretching it taut to make sure there is no negative buckling that can cause what you are seeing. Then try a print wide open and stopped down as well. If both are sharp you are OK. If the neg is flat the image should be sharp all the way across the print. If there is a difference in sharpness as you stop down, use this to tell what f/stop is sharpest with your lens. It sounds as if you are a candidate for a glass carrier to make sure the negatives are flat. If you do this and find the unsharpness persists it could be the lens. Even major brands have problems at times. If it is the lens you can probably send it in for service to Nikon, but a good option may be to call Steve Grimes (S.K. Grimes, machinist to photograhers)and find what he will charge to check it out & make sure it is as good as can be. (may even have to remount it in a custom machined barrel for you). If anyone can fix a problem with alignment, Grimes can do so and probably better than Nikon.

-- Dan Smith (shooter@brigham.net), May 17, 2001.

Ed, one thing I didn't mention in my first post on this was that an 80 is not really adequate for enlarging a 6x7 negative size. You need a 90 or the 105 you want. If you can, borrow or try out a 90 and/or 105 & see what difference that makes. Both with a negative stretched flat or between glass and one with the negative carrier you currently use.

-- Dan Smith (shooter@brigham.net), May 17, 2001.

The 80mm 5.6 El-Nikor IS adequate for 6x7 if you stop down to f/11. Also, there is no light fall-off (due to the lens). I find the El- Nikkor 80mm to be adequately sharp on the edges for 6x7, but obviously a 105mm would have better edge sharpness.

-- Mike Feldman (mfeldman@qwest.net), May 18, 2001.

Ed, one obvious question nobody addressed, are you focusing with the lens wide open? many times I see this in people prints, the center sharp and the rest out of focus, when I asked them what apperture they used they say " ah, lens wide open to get less printing time" Maybe all you need to do focus on the center, and then stop down the lens, to maybe the middle apperture. You might find is all you needed.

-- Jorge Gasteazoro (jorgegm@worldnet.att.net), June 01, 2001.