Uneven illumination - Durst 138s

[mbuonocore]

Hello,

I have a Durst 138s and am having problems with uneven illumination. I keep the enlarger aligned with a Versalab alignment tool, and am judging the illumination with a Darkroom Automations enlarging meter. I have done measurements with both my glass carrier, and the metal inserts. I have one of the original Durst opal bulbs. I am using the correct condensors, and have made sure they are clean. I also have the LACALO heat absorbing glass, and it is clean as well.

I understand that a perfectly even illumination is not possible, although I would like to get as close as I can. I was printing a negative with a very even sky, and noticed the problem.

According to my meter, the right and left sides and "front" (closest to me) recieve 1/10th of a stop less light than the center. I can live with that. The "back" (the side closest to the enlarger column) recieves 3/10th of a stop less light.

As I've mentioned, the enlarger is correctly aligned, and there is no obvious light leaks (I particularly checked around the lens board). I've centered the light bulb using the controls; I haven't found the the backwards and forwards movements has much practical effect on the issue. Side to side does, of course.

Does anyone have any suggestions on how I can correct this?

Thanks!

[ic-racer]

Which lens and magnification ratio? The condenser recommendations don't always work out well if the lens is closer or farther from the negative than expected.
When you say you have aligned the enlarger do you mean that you are sure the condenser optical system, the negative, and the lens are all aligned to their optical or geometric centers? I'd go back over that and check as it seems on of the three items is out of place.

[Jac@stafford.net]

Silly question: are the condensers properly spaced?

[Drew Wiley]

Don't expect enlarging meters to be accurate off-axis. They have cosine issues. A better way to spot illumination error is to use a hard grade of black and white paper and strongly develop it. But ideally, the degree of diffusion in an enlarger has to be matched to the lens you use, at a typical usage f-stop. The wider the
angle of the lens, the worse the falloff. I would recommend a custom ground diffuser. If you are unable to do this, contact someone like Glennview.

[Michael Kadillak]

Are the condensers properly aligned? I have a Durst 138S and if you put the condensers in incorrectly, I could see the problem you are describing. If that is not the problem then it must be the bulb. I have been told by Durst reps years ago that the Opal bulbs were not expensive when they were produced ($2). The fact that Durst threw away 99% because of uneven produced illumination when tested caused the price for the bulbs sold with the Durst name on them to be $100 each.

[mbuonocore]

ic-racer: I'm using a Schneider Componon-S 80mm f4 lens, printing 6x6 negs and enlarging them to 9.5" x 9.5". I'm using the 130 condensor on the bottom and the 200 on top. They are facing the correct way, convex side to convex side.

Insofar as aligning the enlarger, I mean that the lens stage and the negative stage are aligned. I've never thought to check the condensor stage. Maybe that's something I can do when i get down to the darkroom. I wonder how I can do it with the Versalab?

For the record, I am testing the eveness of illumination using the lens at f5.6 or 8 - perhaps I should look at other apertures to see if there's a difference. I hadn't thought to do that.

Drew: thanks for the insight. I'll do as you say and overdevelop a grade 5 print and look to see what the effect is. That's not something I have done.

Thanks!

[mbuonocore]

Michael: How does one align the condensors? They are facing the correct way. Are you suggesting to shim them, or is there a mechanism I can use?

[Bernice Loui]

What type of lamp is being used in the lamp house?

There is a lamp height, distance to condenser and left to right adjustment for the lamp, have these adjustments been done to even out the illumination?


Bernice

Hello,

I have a Durst 138s and am having problems with uneven illumination. I keep the enlarger aligned with a Versalab alignment tool, and am judging the illumination with a Darkroom Automations enlarging meter. I have done measurements with both my glass carrier, and the metal inserts. I have one of the original Durst opal bulbs. I am using the correct condensors, and have made sure they are clean. I also have the LACALO heat absorbing glass, and it is clean as well.

I understand that a perfectly even illumination is not possible, although I would like to get as close as I can. I was printing a negative with a very even sky, and noticed the problem.

According to my meter, the right and left sides and "front" (closest to me) recieve 1/10th of a stop less light than the center. I can live with that. The "back" (the side closest to the enlarger column) recieves 3/10th of a stop less light.

As I've mentioned, the enlarger is correctly aligned, and there is no obvious light leaks (I particularly checked around the lens board). I've centered the light bulb using the controls; I haven't found the the backwards and forwards movements has much practical effect on the issue. Side to side does, of course.

Does anyone have any suggestions on how I can correct this?

Thanks!

[Luis-F-S]

Silly question, can you see it in a print? If not, what difference does it make? The illumination of the L138 condensers is about as good as it gets. Try raising/lowering the Thorn bulb slightly and see if it makes a difference. Hard to believe it's off with just a 6x6 negative. What about 4x5 / 5x7? Since the lens stopped down 2 stops try printing with a 100mm lens see if it's any better. Medium format is nowhere near the maximum capabilities of that enlarger. I've used a 5x7 Durst for 25 years and have never noticed any uneven lighting. L

[Emmanuel BIGLER]

receive 1/10th of a stop less light than the center. I can live with that. The "back" (the side closest to the enlarger column) receives 3/10th of a stop less light.

Hello from France.

If I understand well, you have a small un-balanced illumination with 0.1 f-stop on one side and 0.4 on the other side.
This is not too bad if you consider that the simplest model of cos⁴(theta) law for the illumination distribution yields about 0.3 f-stop with respect to the center when you are at 20° off-axis with a standerd lens (focal length = diagonal).
With a square image format and a lens of focal length equal to the diagonal of the format, this angle is reached in the middle of the edges, i.e. exactly where you find the un-balanced
illumination problem. e.g. 56mm/2 = 28 mm off-center, lens focal length 80 mm, the angle is about 19.3° and cos⁴(19.3°)~=0.79, this corresponds to-0.33 f-stop .. in theory.
Well the actual angle might differ, but to me 0.1 f-stop on the sides of thei mage would be a miracle if it was evenly distributed !

So instead of 0.3 - 0.3 you find 0.1 0.4, you are really close to the absolute limits; with a small residual dissymetry to be investigated.