Durst L1200 (condensor) & uneven illumination

[Cor]

Hi,

I have been using a Durst L1200 Laborator with a condensor for about 15 years now (35 mm upto 4*5 inch). I have always been quite happy with this enlarger. In the past I have checked the illumination and noticed it was a bit off.

Recently I looked in more detail: I used the correct condensors and lens (80mm), focused a 6*6 negative , removed the negative, and used a RH Zonemaster II in the density mode to check the illumination on the base board.

I now noticed that the corners and the side were quite off. The L1200 has on the side 2 knobs for the lamp postion, and the lamp housing can be moved in and out as wll.

The best I could do is roughly the following:

0.15 0.08 0.16
0.08 0.00 0.07
0.17 0.06 0.12

(so the middle put on zero, the other figures representing the very corners and sides). Initially I mistrust the Zonmaster, I mean 0.1 means 1/3 stop and 0.15 1/2 a stop.

I find this alot, OTOH I never realy noticed this when printing (probably tells that I am not that a good printer I guess..

So I checked by exposing a grade 5 piece of paper to about middle grey, and sure enough it showed the same effect, (although I am not sure if I realy see that half a stop difference) the corners beeing somewhat lighter.

Somehow I find this dissapointing for such a sturdy enlarge of great reputation, you might say why worry if you're happy with prints, but it is nagging..

So I tried to diffuse the light btween the bulb (a guinine Phillips Photocresenta, 75 watt) and the condensor with some milky plastic, paper, and even a piece of groundglass. It did not help, and even made matters worse.

Aother solution would be a diffusion head, a CLS501.
I happen to own such a head, but I was not too happy with it for various reasons.

So any suggestions on improving matters will be greatly appreciated!

Best,

Cor

 

[dancqu]

0.08 0.00 0.07... so the middle put on zero, the
other figures representing the very corners ;Best,Cor

Is a quarter stop off so bad? I do know that if
I change exposure from 8 to 10 seconds, a quarter
stop, the difference is noticeable. So maybe
not so good. Dan

 

[dancqu]

So any suggestions on improving matters will
be greatly appreciated! Best, Cor

Recalling, I had a similar problem some years ago.
Prints made with my Meopta dichro exhibited light
fall-off towards center. So I made blank paper tests.
Sure enough the prints were lighter in center. Turns
out some big brand bottom diffusers also are over
dense towards center.

Quit likely the tests were made with lens in place.
Lenses invariably exhibit light fall-off at their perimeter.
In your case with condensers in place, with their focused
beam of light, tests I think may be made minus lens. Dan

 

[Cor]

I am answering my own question here: somebody pointed out that my observed light fall off can be explained by the light fall off of my enlarger lens (Schneider-Kreuznach Comonon-S), it seems that all enlarger lenses show light fall off at the corners, and worst at the low f numbers (see graphs on the Schneider-Kreuznach site). My observations fit with these graphs, and the readings get worse when stopping up from f11 to f4.

Luckily this light fall of is more or less canceled out by the light fall of of my taking lenses (if you shoot negatives off course). Perhaps thats why I never really noticed this phenomena in my prints.

Best,

Cor

Hi,

I have been using a Durst L1200 Laborator with a condensor for about 15 years now (35 mm upto 4*5 inch). I have always been quite happy with this enlarger. In the past I have checked the illumination and noticed it was a bit off.

Recently I looked in more detail: I used the correct condensors and lens (80mm), focused a 6*6 negative , removed the negative, and used a RH Zonemaster II in the density mode to check the illumination on the base board.

I now noticed that the corners and the side were quite off. The L1200 has on the side 2 knobs for the lamp postion, and the lamp housing can be moved in and out as wll.

The best I could do is roughly the following:

0.15 0.08 0.16
0.08 0.00 0.07
0.17 0.06 0.12

(so the middle put on zero, the other figures representing the very corners and sides). Initially I mistrust the Zonmaster, I mean 0.1 means 1/3 stop and 0.15 1/2 a stop.

I find this alot, OTOH I never realy noticed this when printing (probably tells that I am not that a good printer I guess..

So I checked by exposing a grade 5 piece of paper to about middle grey, and sure enough it showed the same effect, (although I am not sure if I realy see that half a stop difference) the corners beeing somewhat lighter.

Somehow I find this dissapointing for such a sturdy enlarge of great reputation, you might say why worry if you're happy with prints, but it is nagging..

So I tried to diffuse the light btween the bulb (a guinine Phillips Photocresenta, 75 watt) and the condensor with some milky plastic, paper, and even a piece of groundglass. It did not help, and even made matters worse.

Aother solution would be a diffusion head, a CLS501.
I happen to own such a head, but I was not too happy with it for various reasons.

So any suggestions on improving matters will be greatly appreciated!

Best,

Cor