Velostigmat Series II Diffusion Question

[Fotoguy20d]

I'd like to understand how the diffusion feature works on these lenses. Is it changing the gap between the front pair? between the front pair and the rear doublet? Both? Presumably, when I'm trying to change the diffusion, I'll be shooting wide open?

Reason I ask is I have a 7" which of course lacks that feature. I was able to rig it so the rear glass of the front pair is linked to the nut which retains it. So, I can dial in the separation between the front pair. The front/rear can be controlled easily enough just by varying how far on the front element is threaded to the shutter. Just looking on the ground glass, it seemed like there was only a little softness achieved by changing the spacing within the front pair. I'd rather have a better understanding before I start blowing through film experimenting.

Thanks,
Dan

[David Aimone]

Someone will correct me if I'm wrong, but I believe the rear element of the front group maintains its distance from the rear group and shutter, and the front element of the front group moves forward, farther from the rear element of the front group, to produce more diffusion.

In other words, on the 9.5" lens that I have, the front element in the front group partially unscrews to produce more diffusion. The mod was to remove an internal screw that kept the front element from being unscrewed more than one rotation. The rear element of the front group maintains it's distance from everything behind it.

On the 7.5" version, I think you have to find a way to move the front element forward, while maintaining the same distance from the shutter of the rear element of the front group.

Whoa! It's not easy describing this sort of things in words.

[Fotoguy20d]

David,

Fairly straight forward if I understand you correctly, but difficult to implement in practice, since I can only move the second glass back from the front one, and not move the front one forward. But, I suppose, assuming the threads are the same pitch, for each rotation I unscrew the back from the front, I need to then unscrew the front barrel by one rotation relative to the shutter (and therefore, the rear). Perhaps not so difficult to implement after all.

Dan