How do you tell the size

[jtomasella]

Looking at Petzval lenses, how do you tell the size in mm. If the lens is physically 6 1/2" long.

[Molli]

1 inch = 25.4mm if that helps at all. While I know some conversions off by heart between imperial and metric, Celsius and Fahrenheit, I confess to using a conversion app on my phone on the fly when I'm not at home or looking things up in one of my old Ilford manuals, particularly for very old weight conversions when I am.

[maltfalc]

you don't.

[mhayashi]

I think it depends on whether you measure the height of the petzval lenses including the hood or not.
The hood size varies depending on makers and some hoods are lost or cut short over the time.
So I suggest you to measure the rear element diameter to guess the focal length that you imply by saying the size.
There is a Dan’s thread in FB that you can refer to. Search by the words “rear element diameter petzval dan”.

[Jody_S]

I have 3 identical-looking small petzvals that range from 6" to 12" EFL. I can't tell them apart, I have to keep tags on them.

[CCHarrison]

I am "Dan" see attached