Vintage SOM Berthiot 25mm f1.8 C mount lens, front thread?

[Mark Eastman]

[ old obscure French lens department ]

I have a vintage SOM Berthiot 25mm c mount lens and am having trouble identifying the front thread for a filter. It's not 38mm or 37.5mm does anyone know SOM Berthiot's front threading on these lenses? Photo below.

Thank you,

Mark

Mark Eastman
Designer/Photographer
Palo Alto, CA USA

[Mark Eastman]

It's not 38mm either (somewhere between 37.5mm and 38mm)

Thanks

Mark

Mark Eastman
Designer / Photographer
Palo Alto, CA USA

[Simon Wyss]

It’s the ISO II M 38; pitch 0,5 mm. You’ll never measure the major diameter of an internal thread with home instruments. What you can measure is the minor diameter which should be between 37,459 mm and 37,599 mm.

[Simon Wyss]

The optics, by the way, are that of the 1924 Perlynx of Hermagis. Berthiot bought Hermagis in 1934 and continued the dialytic four-elements design as Cinor B, later as Lytar. Thanks to the additional strongly positive rearmost element to a triplet the field is flat and evenly illuminated. In short,  a good lens for a decent price

[Mark Eastman]

Thank you Simon for both the technical and historical details on this lens!

Kindest regards,

Mark

Mark Eastman

Designer / Photographer

Palo Alto, CA USA