[LargeFormat] Recementing Lenses - Part 3

[Richard Knoppow]

trade name "Velostigmat". Wollensak was one of the first 
companies to coat lenses and to use synthetic cement. 
Mechanically, their lenses are very well made.

Both the camera and lens were probably military issue. The 
specification for the lens is probably the same one that 
applied to the Goerz Dagor and Gundlach-Manhattan 
Turner-Reich lens, maybe others. This lens is a Convertible 
Protar type with cell focal lengths of 20" and 25". It is an 
excellent lens, quite sharp and with relatively little zonal 
spherical aberration, a property of all double meniscus 
lenses. It covers 8x10 sharp to the corners at about f/16 
although its optimum stop is probably f/22. The individual 
cells have very good performance with relatively little 
color fringing. They are sharp to the edges at around f/32, 
which is pretty good for this type of lens. Dagor halves 
should be stopped down to f/45.

It is very much superior to the T-R lens, not really 
surfriding.

This lens is in a Wollensak Alphax shutter. These are 
excellent large size shutters, IMO better than the 
equivalent Ilex types.

Wollensak lenses are of variable quality, due mostly, I 
think, to design problems, but the

company was capable of making excellent lenses on occasion, 
this seems to be one of them. Wollensak shutters have always 
been excellent.




A note on centering.



When a lens is made it is centered after grinding and 
polishing. Because the surfaces of most lenses are segments 
of a sphere they will automatically center around the axis 
when clamped between two ring shaped surfaces.

Lenses to be cemented rely on the edge of the lens as the 
reference surface for location of the elements during 
cementing. They require more precise centering than single 
elements. The centering method is a fairly simple one. The 
element is held on the end of a rotating tube with s 
flexible cement, like pitch. A small light source is shined 
on its surface and the reflection examined through a 
telescope. If the lens is not centered on its axis the two 
reflections will appear to orbit each other. The lens is 
repositioned on the tube until the two reflections are 
absolutely stationary. Then the lens is clamped by a second 
tube above it and the edge ground exactly parallel and 
concentric to the optical axis. For cemented elements the 
absolute diameter of the lens is important because the edges 
are what are used to hold the assembled elements in place 
during cementing.

In some lenses the elements are not of the same diameter. 
The Schneider Angulon is an example of such a lens. When 
lenses of this sort are cemented the edges must be held by a 
special fixture. Its possible to center the two elements in 
a way similar to that used for the original centering and 
edging but a process like that would be uneconomical in 
production. I don't know of a practical method of 
recementing unequal diameter lenses on a small scale.

In summary, I am glad I undertook this project. It was quite 
frustrating at times but I now have the knowledge to 
recement most lenses, which may be valuable in the future. 
It was certainly worth the effort.

I've probably left something out. I will be glad to answer 
any question, provided I know the answers.

Again, I thank John Handly for his advise and support.

Summers Optical is at: 
(http://www.emsdiasum.com/Summers/optical/cements/default.htm)

The Summers site has a very good primer on lens cementing 
including preparation and full technical details on their 
various cements.

The letter referring to Methylene Chloride is at: 
(http://astro.umsystem.edu/atm/ARCHIVES/AUG99/msg00231.html)

July 20, 2005

---
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com