setting Dagor cell spacing?

[John Schneider]

I have a set of 19" Dagor cells (75x,xxx) that I restored, and another 19" (22x,xxx) set that Steve Grimes mounted into an Ilex 5 (*very* nice work) long before I bought it. When it comes to putting the 75x cells into a shutter, how should I set the spacing?

Can I assume that if I measure the external length of the 22x cells in shutter, going from outer convex surface to outer convex surface (using a surface plate, gauge blocks, and height gauge to measure) that I'll have the correct measurement for the other set of cells?

This is the easiest measurement by far, but I don't know if the Dagor design changed enough over those decades that the spacing was changed. Would taking more measurements to get the internal, minimum cell spacing (from inner concave surface to inner concave surface) be necessary?

[Steve Hamley]

I think the shutter iris needs to be in the same relative position to the cells as in the original barrel, so the answer would be "not necessarily". I'd want to make sure the 75xxx cells were the same distance from the iris as the 22xxxx cells as a starting point.

Cheers, Steve

[Dan Fromm]

You could do worse than to go to dioptrique.info, read the f/6.8 Dagor prescriptions and space the cells accordingly. They don't vary much from patent to patent. Also check the f/6.8 Beryl prescriptions, they are very similar to Goerz Dagors.

[John Schneider]

I used a variety of gauge blocks and balls and was able to measure the overall thickness of all the cells. The newer (75x) front cell was 0.003 shorter than the corresponding 22x front cell, but the rear 75x cell was 0.003 longer. So, in conclusion, all I have to do is match the overall lengths of both sets of cells (measuring from convex lens surface to convex lens surface).

[c.d.ewen]

John:

Do like I did: I bought a barrel without an iris, so I bought a second one to measure. What could possibly go wrong?

Charley

[mrossano]

Once you get the spacing set to a nominal distance, you might consider sending the assembly to Schneider Corporation of America's service department. The last time I spoke with them (two years ago), the charge to mount a lens on a projector and set the cell spacing to optimum (to within approx. 0.0005") was about $50 plus postage. This assumes that they have the proper sized shims for your lens barrel or shutter.