Velostigmat series II diffusion?

I just got a 12 inch Velostigmat series II with the front cell diffusion ring. In testing the lens, it seems that cranking the diffusion ring changes the focus and when I refocus, I don't see diffusion, just a changed depth of field. Has anyone had similar observations?

Hi-
Are you using it at the "soft" apertures- i.e., wide open or nearly so? After about f/8, it becomes pretty sharp...
Russ

athanasius80 said:

I just got a 12 inch Velostigmat series II with the front cell diffusion ring. In testing the lens, it seems that cranking the diffusion ring changes the focus and when I refocus, I don't see diffusion, just a changed depth of field. Has anyone had similar observations?

Click to expand...

with my vitax, the diffusion ring allows you to stop the lens down a bit, and still achieve a soft image. it is a "defocus" knob and changes the placement of rear elements ... try shooting the lens wide open with the ring at different settings, and then try stopping the lens down, and re-set the diffusion ring - don't re-focus though

i love being able to stop down to f16 and crank the de-focus knob to 3 or so, it gives a beautiful effect ...

-john

Followup: Thanks guys. Someone taped the diffusion ring somewhere in the past and I'm still looking for the number 5 on the ring and the number line on the barrel. Also, Its a 8x10 Velostigmat, and I don't have an 8x10 camera yet. I was just surprised at its behaviour on the ground glass and wanted to know what I might be doing wrong with my 5x7. Thanks for the ideas, keep 'em coming.

My 360mm Universal Heliar is the same - the diffusion effect is much less at smaller apertires, and "dialing in" diffusion changes the focal length and therefore focus.

Steve