"JB" wrote in message
...
In article
.com,
murrayatuptowngallery
wrote:

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 purchased a G Claron 150 from Chuck Farmer at The F
Stops Here about
15 years ago. It's a great lens under certain light
conditions. I used
it configured as 150 and as 300. In the 300 configuration,
you need an
adaptor that will permit the mounting of a yellow filter,
to replace the
front element.

Sorry, but I don't have the filter dimensions / specs at
hand, but
34.5mm pops up as the correct fit.

The most critical aspect of using the 150 as a 300mm is
the reality of
single-coating on the G-Claron lens, which after all was
intended to be
used as a flat-field repro lens. Attach a lens shade to
the yellow
filter and never shoot even obliquely into sun.

With yellow filter and lens shade the G-Claron 150
configured for 300
produces wonderful, sharp images. It's a great lens.

Regarding Ron Wisner, I used the G-Claron on his Technical
Field camera
for five years. Great combination.

- Jan Becket, Honolulu

The problem with all convertible lenses is that when a
single cell is used one loses the automatic corrections
introduced by symmetry or even semi symmetry. Symmetry
corrects the three lateral aberrations: lateral color, coma,
geometric distortion. Some convertible lenses, for instance
the Zeiss Convertible Protar, have some correction for coma
in the individual cells but most do not. For instance, the
Dagor, while patented and sold as a convertible really is
not because the individual cells have a lot of coma so work
only as very small stops (around f/45). The combined lens
has very little coma. While the cancellation is maximum
where the entire optical system is symmetrical (equal object
and image sizes) it is considerable even at infinity.
The Plasmat is really a derivation of the Dagor where
the inside cemented element is broken free and air-spaced.
This gives the designer an additional surface and a spacing.
While the Dagor has considerable uncorrected spherical
aberration the Plasmat has very little. The design was known
for decades but not used widely because of excessive flare
where not coated. The availability of economical coating
methods following WW-2 changed the whole approach to lens
design making it possible to use air-spaced elements in
place of the previously used cemented elements. This
resulted in the practicallity of using some excellent
designs previously not used due to excessive flare and a
considerable improvement in lens performance.
I don't know why the lens would need a color filter.
Perhaps its chromatic correction depends on its symmetry,
certainly its lateral color correction does. Filters have no
effect whatever on flare, other to increase it if the
filters are not coated.
Process lenses when used for their original purpose in
making photo-mechanical half tone plates, do not really need
to be coated since the flare can be corrected by a small
change in exposure.


--
--
Richard Knoppow
Los Angeles, CA, USA