How to tell a RR from a petzval?

[RobertP]

The 24" Derogy petzval arrived today. But It looks almost just like my Dallmeyer 25" RR. Both elements of the Derogy both front and back seem to be the exact same size and they both appear to be cemented. This is noticable by the separation that most of these brass canons have. Can someone confirm if this is a rapid rectilinear or a petzval if I send them pics of the lens? Thank you in advance. I really don't need 2 rapid rectlinears of the same focal length. Robert

 

[David H. Bebbington]

A quick look at a reference book indicates:
Petzval: 4 elements, front group of 2 elements cemented, convex outside surface, concave inside surface, rear group air spaced, both external surfaces convex.
Rapid Rectilinear: 4 elements, 2 identical cemented groups of 2 elements each, one external surface convex, one external surface concave (the concave surfaces facing in towards the iris diaphragm).

Does this help?

Regards,

David

 

[RobertP]

Thanks David, that confirms my suspicions. I'm glad you pointed out the concaving surfaces pointing inward toward the diaphragm. It is a Rapid Rectilinear and it has been returned. Thanks, Robert

 

[David H. Bebbington]

Glad to have been of help, Robert - it was hard to judge from the pictures you sent, but the lens just looked like an RR!

Regards,

David