"murrayatuptowngallery"
wrote in message
oups.com...
I just read a 10 year old post about using a G-Claron as a
'pseudo-
convertible' by using one cell; performance would probably
be better
than 'vintage' convertibles.

I had previously wondered about splitting plasmats but
wrote it off as
'sacrilege' on already decent lenses, but seeing the post
made me
realize it's been done.

Anyone actually try this with a G-Claron 150 and does it
have any
unique qualities, or just 'work'.

Thanks

Murray


I think, without looking it up, that the G-Claron is a
Plasmat type. These will work as convertibles. The best of
the old convertibles was the Zeiss Convertible Protar
sometimes called a Double Protar. The difference is that
these lenses are individually corrected for coma where some
other convertible lenses, notably the Goerz Dagor and Dogmar
are not. This allows the Protar to be used at larger stops
with reasonable sharpness in the corners. Dagor cells must
be stopped down to around f/45 to get rid of enough coma to
be sharp away from the center. Protars are OK at f/36 or
maybe even a little wider.
Nearly any symmetrical lens can be used as a
convertible but, as noted for the Dagor, some aberrations,
especially those corrected by the symmetry, will be present.
My experience with the convertible type of Symmar is
that they perform about as well as a Convertible Protar, but
not better. Actually, I think the correction of the
individual Protar cells is a little better.
I don't know if the early Plasmat type Symmar (the
first lens of this name was a Dagor type) was specifically
corrected for coma but it probably was. Later ones are
corrected for best performance in a complete lens but single
cells probably perform pretty well when stopped down enough.
This should apply to the G-Claron.
FWIW, the Plasmat was derived from the Dagor by
air-spacing one of the elements. I has similar performance
but much less zonal spherical aberration, an inherent fault
of the Dagor type. Plasmats can also be corrected for
astigmatism to a very high degree. Like the Dagor they are
essentially wide angle lenses although most have perhaps
about 10degrees less coverage than an equivalent Dagor. The
single cells of convertible lenses of all types have quit
narrow coverage, the plate size being about the same as the
assembled lens.


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Richard Knoppow
Los Angeles, CA, USA