Help me understand the Durst condenser system on the 138

I'm wondering if anyone can help me work through the nuts and bolts of the Durst Latico condenser system on their 138, and I suppose 184, model.

I know I can refer to the charts, which I do. I know the goal is attaining bright and even illumination.

What I'm wondering is why the condenser combinations are what they are? Why do I use a 130/85 combination with a 50mm lens for a 35mm negative? What is the math behind it? Is there any?

I'm working towards something, and while this may seem unnecessary, I think it will help a great deal if I can understand the basics.

Thanks!

Without maths:

The light of the lamp has to be projected into the enlarging lens. To be precice at its nodal point.

Thus by this the FL of the enlarging lens plays its role. But also the working aperture.
To reduce sherical abberationn a condensor lens-pair is used.
Also the length of the lamp filament is of effect.
To erhance lighting of the main condersers a third condenser is used.

Changing image scale will neccessitate to re-focus and this necessitates to re-adjust the lighting system.

This applies to near-nodal light sources. The larger the light source gets the less critical the optical system gets.



The math basically is the lens formula.
But one has to take into account that there are two objects and two images:
the lamp filament and the film and their image in the lens resp. on the paper

Thank you AgX, for taking the time time explain that. I really am fascinated by the optics involved. I sometimes use a point light source, and I am getting quite familiar with the idea of focusing the lamp filament with changes in magnification / image scale. I see how this differs from using a more diffuse source like an opal bulb where bulb placement is less critical.

But what I am still having a problem understanding is why - besides consulting the Durst information - would I use a 130/85 condenser combination with a 50mm lens for a 35mm negative. Or a 240/240 combination with a 150mm lens and a 4x5 negative. I just can't connect the dots.

The light cone that is projected through the film into the enlarging-lens should ideally be the same as the cone from connecting object-points (at the film) with the center of the enlarging lens.
By this it should be clear that there is a connection between the FLs of the condenser and the enlarging-lens, resp. between their extensions.

Marco just do it! U can also try different condenser combinations with lenses. I believe the charts are set up for maximum light and even illumination with a specific lens. Did U get the condenser set at auction? L

In addition to the other posts aboe, note that the condensers project an image of the dome of the lamp. If that is too small you won't get even illumination. This is more of a concern with 4x5" and 5x7" negatives.

The math basically is combining two lens-equations: that of the enlarger-lens and that of the condensor.

You would start at a given format, an appropriate focal lenght for the enlarging-lens and an image scale of your choice. From that you calculate the (bellows-)extention of the enlarging lens.
From that you can calculate the FL length of the condensor, its position to the lens and the position of the lamp.

Or something like that...

Thanks for the input everyone. I'm still sort a few steps a way from really understanding all this, but that's life, isn't it?

The reason for my wanting to get a handle on this is because I'm having problems focusing the filament of my point light source with *most* combinations listed on the Durst charts. I am fortunate that one of the combinations that I can get to work perfectly is for my 6x6 negs - which is the bulk of my work. I'm thrilled with the look I'm getting.

I have a comprehensive set of coated condesers, and a lot of lenses to choose from. The most simple combination - and the one I'm most interested in - is the 130/85 plus a Rodenstock or Schneider 50mm lens. I have all of the above (plus a Nikon 50mm and a Leitz 50mm) and I can't get the filament to focus on the easel within the stated magnification range with any combination. It "floats" maybe 6" to 9" above the easel (I can see it with a piece of white mat board). I can get nice even light, which is good, but I should be able to get optimum collimation (is that a word?), and I can't.

So here's the thing: I don't think the Durst information is wrong. No chance. I figure *I'm* wrong, and I just can't see where I'm slipping up. It's a fun winter project, and I think it'll just take one piece of information to make everything click into place.

Oh, and Luis, no, I didn't buy that set. I have a spare set of uncoated condensers as it is, and I can't stomach the idea of more junk around the darkroom!

When installing a new small (point light) lamp, you will have to adjust it in three directions. Once done that you only have to readjust the lamp when changing the image scale, and then only in one direction, along the optical axis.

AgX, I actually find focusing the lamp very intuitive. The system on the durst 138s is so easy to use. With the example of my apo-rodagon 90mm lens and the 200/130 condenser combination, I can focus the lamp in 20 seconds. The light is perfectly even, and it's a joy. The filament is perfectly evident on the easel. This isn't the case with other lenses : the filament is nowhere near the easel. It's up above the easel, and no matter where I place the bulb using the movements of the lamp, I cannot get the filament focused.

I can't quite figure what I'm doing wrong, but there's obviously something.

The filament image should not be on the easel, but the film image should be.

The image of the filaments of an enlarger or projector should be placed into the aperture of the enlarging/projecting lens.

http://www.durst-pro-usa.com/world_images/theofcon.htm

gives some detailed explanations.