Half Aplanat usage

[Neil Purling]

Has anyone experimented with using the rear half of a Aplanat or Rapid Rectilinear?
I wonder what multiplication of focal length you'd get and the effect on control of optical aberrations. I got a 6" Aplanat in a Copal #1. I figure i'd lose a stop and a half speed or thereabouts.

[jnantz]

hi neil

i often use half of a rapid rectalinear ..
the focal length is around 1.75x larger than the number printed
on the dial. so a stop + 1/2 is a pretty good guess ;)
i guess you have to guesstimate the focal length ...

have fun ;)
john

[Toyon]

Wollensak, one of the last commercial producers of RR lenses (they produced beautifully sharp versions) published the focal lengths of the rear and front elements used alone. You'll see that they roughly double the focal length. A good starting point is that the light is reduced by around 1.5 stops.

Link:

http://cameraeccentric.com/html/info/wollensak_13.html