Help with brass lens

[elrint]

I bought a 19th-century camera and was wondering if anyone by chance had some helpful info about the lens? I've searched from links here and my own research and not making much progress, and hoping someone here may know something. It has '18491 / C. Berthiot Fabricant Opticien Paris / No. 3' engraved. Would also like to know aperture size since it is not listed and I've tried doing the focal length/diameter but the math just doesn't seem right...any help is appreciated.
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[Two23]

It looks like a rapid rectilinear lens. Set flat on a table with bright light above shining directly into it. Slowly raise the lens watching the spot of light focus below it. When the spot is sharpest measure from the spot to the middle of the lens. That's roughly the focal length. I would guess it will be 8 to 12 inches.


Kent in SD

[elrint]

Yeah I tried that but I did it in a large room against a wall until the spot of light was sharp..but i used a flashlight so not sure if that messed up the test or not..because the spot was sharp on the wall and it was about 10 feet away haha. But I'll try again tomorrow using natural light from window and a smaller room and try it again.

[Two23]

I think you'll find the lens is likely f8 too.


Kent in SD

[Philippe Grunchec]

Patrice H. Pont & Jean-Loup Princelle:

http://www.lereve-edition.fr/en/dossiers-collectors-1-a-10/berthiot-dossier-collector

[alex from holland]

http://https://www.facebook.com/100000725324881/posts/1191236357577197/? (http://www.facebook.com/100000725324881/posts/1191236357577197/?)
Here you can see how to measure it quite accurate.

[elrint]

Yah it's about an F8 and then Waterhouse stop makes it approximately f18.