On 2/12/2011 12:33 PM, Jean-David Beyer wrote:

Cheesehead wrote:
I'm not absolutely certain about the Geronar formula. It is reported
to be a Tessar.
That said, the rear should be a single piece of glass. But it is not.
If it is a cemented piece, then it is not a true Tessar and the
fogging may be in the cement.
That would be bad.
I've not found the formula out there to describe the lens.
In the mean time I've been searching for a replacement rear cell.

I thought a Tessar had four elements, the rearmost was a cemented doublet.

Lot of confusion in this thread..

A 210mm f6.8 Geronar -IS- a triplet. The rodenstock sales booklet at the
time these were sold stated this.

http://www.bhphotovideo.com/c/produc...nar_Copal.html

A tessar has a cemented doublet but this lens isn't a tessar.

http://en.wikipedia.org/wiki/Tessar

Someone along the way may have swapped out the rear cell from some other
lens into the sample he has or something odd, I have no idea. Maybe he
is confusing a coating problem with fogging inside the cell? I have no
idea on that either.. But the rear cell on a normal 210mm f6.8 geronar
is a single piece of glass.

Stephey