Derogy petzval...with or with BIS

[Jon Wilson]

I have notice a couple of petzvals on this and other sites which describe Derogy No. 5 petzval lens, same FL, but one has BIS after its number. Is a there a practical difference between those lens or is one just a new "model"? Ford Fairlane vs Edsel? Eddie?

Thanks....BTW...how all have a Happy Holidays!

Jon

[Steven Tribe]

Bis is usually to denote a size (focal length) that has been introduced after the original manufacturers' model range was fixed. That is, a size 5 bis would be intermediate between sizes 5 and 6. There may be other design differences rather than FL in specific cases.

[Jon Wilson]

Thanks Steve. That is very interesting and helpful. Jon

[Steven Tribe]

The system is mostly used on German and French lenses.
Here is an example I have on a Berthiot trousse (casket) set.

[Ramiro Elena]

What would a 5-Bis be in FL? I can't seem to find anything on the VM.