I have several older chrome (and black) lenses where the heads detatch from the focusing barrels. I'm assuming this for use on the Viso's (21 SA and 50' black 'cron separate too; don't know why).

After separating them in order for easy access to the rear element and focusing guts for cleaning, there is some hand engraved (read scratched) numbers on the head referring to the actual focal length of the optics. All the numbers match the number properly engraved on each's focus mount near infinity, except my 50 'cron. I don't have the numbers with me now.

The focus mount might have been damaged and changed on this lens, where the lens group survives. Will this affect proper focusing? Are mounts matched to the lenses like 'blad magazines? Or, is the engraving just to inform users the true focal length, and if so, of what use is this information?

-- chris chen (chrischen@msn.com), February 28, 2002

Answers

Hello Chris. To my knowledge detachable lens heads were factory calibrated to their respective focusing mount.Lens heads mounted to unoriginal mounts could introduce focus errors,especially in the 135mm.lenses where a 1/16th extra or lesser turn when seating the head can affect focus,especially close up.

The small number near the infinity mark refers to the true focal of the particular lens e.g.19 on the 50mm.equals 51.9mm.

The head of black version 2/ 50mm.'crons actually did unscrew for use,with adaptor, as an enlarging lens on the Focomats. Regards.

-- Sheridan Zantis (albada60@hotmail.com), February 28, 2002.

It is a simple matter to have your lens checked on a collimator. The head of early 50's was meant to detach and re-attach to a copy stand. The 21 S/A might come apart if you unscrew it, but for no other purpose than disassembly.

-- Jay (infinitydt@aol.com), February 28, 2002.