Hello,

first, I want to apologize if I'm posting in the wrong sub forum. I don't really know which one is the right for my kind of "problem". And here it comes:

I have a nice 7x11 Chamonix which I use with a few different lenses. I focus with a loupe on the ground glass, and usually, the part I focus on the glass is the part that is sharp on the plate. As it should be.

Now I have this nice old petzval portrait lens, and whenever I focus correctly on something on the glass, it shows a front focus of about 3 centimetres on the plate/negative.

But only with THIS lens. Where is the problem?! I don't understand?!

If the focal plane would be wrong with my back or the holders, it would be the case with any used lens, not only this certain one?

Can anybody help me? It freaks me out. I don't know how this could happen. How can it be sharp on the ground glass but then be greatly misfocused on the final plate or negative? And it also happens with still motives that can not move, so... it is really weird.

What do you think? Your help is greatly appreciated.

Thank you!

Best regards!