Just to let you know that I've painted the mounting plate (it's more like a ring) matt black.
Not much of a change, but there was a minor improvement (the 'halo' seems dimmer).
So, looks like a case of the condensers not been optimized for 40/50mm lenses (and this is true, since it's a 6x6 enlarger).
Opemus uses both concave and convex mounted lens boards. In the case of a 50mm or smaller focal length, you screw the lens to the concave side of the board. But if the lens is too large, it will screw only partially (if it does at all) into the board, allowing the light to escape around the thread. It happened to me with a Minolta lens, which is a large model. This doesn’t happen with the 75-90mm lenses, as you screw them to the convex side of the lens board, and in this case the lens’ size doesn’t matter.
It's not my case.
The original lens board wouldn't even allow mounting the EL-Nikkors (defective part).
I had to order a custom machined lens board, and the lens screws fully in this one.
As I have always played with Durst enlargers, from 6x6cm to 5x7', soon I learned that one can't fool around with optical geometry (as an engineer, I suppose you know what I mean). With Durst bigger enlargers one needs to check both condensors, their distance from lens, bulb size and position on XYZ space, magnification and, if one really wants to be precise, the filament position inside de bulb. Once it's all set, light distribution is so even as it can be.
Regarding to Opemus III, I could suggest you try both ways: 60mm lens and 1- turning the head for floor projection or 2- using a wood block 10/15cm high under the column, so to allow 30x40cm projection on the board.
BTW, as I'm also brazilian, feel free to e-mail if you want. Certo?
The Opemus III is in phase of replacemet by an Opemus 6 I found in Germany's Ebay and that a friendly soul over there is reshipping it for me (German sellers usually do not ship out of Germany).
That's a nice move and I suppose you'll have a complete set of condensors. But, as far as I remember, Opemus VI shares the same column and you may still have problems with bigger prints. But, as you've seen, that's easy to deal with.