150mm G Claron

The official Schneider quote for the image circle is around 180mm at
infinity and f22.
It said by some this is a conservative figure.
Also how good would the lens be at apertures wider than f22 around infinity?
I normally use f16, f22 and smaller anyway but am curious about the efect on
the edges of the image.

My camera is a 4x5 Crown Graphic which doesnt have mega movements.

The design of all the G Clarons is such that the image circle continues to
increase as the aperture is stopped down. As you probably know, G Clarons
are optimized for 1:1 but Schneider says they work well as general purpose
lenses if stopped down to F22 or smaller. So I've never used mine at
anything wider than F22 but at that aperture and smaller the 150 covers 4x5
with room for movement, the room increases as you stop down below F22, and
the edges are fine. I've owned three G Clarons, the 150, the 210 (which I
use for 8x10), and the 240 (which I also use for 8x10). All have been
excellent lenses and great for backpacking since they're so small.

"Neil Purling" wrote in message
...
The official Schneider quote for the image circle is around 180mm at
infinity and f22.
It said by some this is a conservative figure.
Also how good would the lens be at apertures wider than f22 around
infinity?
I normally use f16, f22 and smaller anyway but am curious about the efect
on
the edges of the image.

My camera is a 4x5 Crown Graphic which doesnt have mega movements.

"Neil Purling" wrote in
message ...
The official Schneider quote for the image circle is
around 180mm at
infinity and f22.
It said by some this is a conservative figure.
Also how good would the lens be at apertures wider than
f22 around infinity?
I normally use f16, f22 and smaller anyway but am curious
about the efect on
the edges of the image.

My camera is a 4x5 Crown Graphic which doesnt have mega
movements.


Schneider has pretty complete data for this lens on its
web site at http://www.schneideroptics.com. There it states
that the lens can be used for infinity focus at f/22 or
smaller and that it will cover 64 degrees without vignetting
at f/16 or smaller. This is equivalent to 187mm for a 150mm
lens at infinity focus.
This is a Plasmat type lens. Plasmats have very good
corrections especially for astigmatism. Most lenses that
increase coverage as they are stopped down do so because
they have a considerable amount of uncorrected spherical
aberration, which is proportional to the stop. A Dagor is
such a lens. Its coverage is about 60 degrees wide open
increasing to around 87 degrees (claimed) at f/45. The
better corrected a lens is the less its coverage will
increase with stopping down. The G-Claron is a very well
corrected lens. The main advantage of stopping down is to
eliminate vignetting by the lens mount. Unless a lens is
especially designed to be used wide open it will probably
mechanically vignette until stopped down about 2 stops, that
is, until the size of the aperture is about half its wide
open value. This varies with the lens. As one moves further
out in the field the aperture needed to avoid vignetting
becomes smaller so there is no fixed value for this. In any
case, it looks like the maximum coverage of the G-Claron is
not very much larger than its wide open coverage. 187mm is
enough to exceed the movements available on a Speed or Crown
Graphic.
As far as the image conjugates are concerned, a lens can
have simultaneous correction for spherical aberration, coma,
and astigmatism at only one object distance. This is chosen
by the designer according to the application for the lens.
Copy and reproduction (process) lenses are generally
optimised at 1:1 magnification which is equivalent to an
object distance equal to the image distance or twice the
focal length. General purpose camera lenses are optimised
for infinity focus (actually more often for around 20 to 50
times the focal length for a better compromise for closer
work). The aberration which is most often observable when a
lens is used far from its optimum object distance is coma.
Coma varies with the stop and is reduced by stopping down so
many process lenses are quite good at infinity provided they
are stopped down enough to get rid of the residual coma. For
a four element lens like the Apochromatic Artar, this is
around f/22 as it is evidently for the G-Claron. Four
element "Dialyte" lenses, like the Artar, are relatively
insensitive to object distance. I don't know about Plasmats,
but, in general, the slower a lens is to begin with the less
sensitive it is to variations in conjugates. This has to do
with the changes in the angle with with the light rays going
through the lens strike the surfaces of the elements. The
narrower this angle is to begin with the less the variation.
So, its greatest for high speed and wide angle lenses, least
for slow, narrow coverage lenses like process lenses.


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Richard Knoppow
Los Angeles, CA, USA