Light corners though "perfect" set-up

[Oband]

I am using a Siriocon 80 condensor with a 80mm lens to do 6x6 b&w negatives on my Durst m605, but I get one corner (in particular) slightly lighten than it should be. This one side and also the other left corner above on the print does not get enough light through the lens. I have tried adjusting the lamp, changing the lamp, different lenses, and different apertures (typically set to f.11). Nothing helps, any advise please?

 

[compur]

It's called light falloff and is usually a lens aberration which is also usually aggravated by using smaller apertures.

What lenses have you tried and why are you shooting at f/11? (optimum apertures are usually 1-2 stops down from wide open).