[LargeFormat] An 11x14 Ektachrome shot with rear half of a Protar VII convertible

[Clive Warren]

At 14:36 13/10/2003 -0700, Richard Knoppow wrote:
snip
>   The "correct" position for either cell is behind the
>diaphragm. The stop position influences coma, spherical
>aberation, and field flatness. However, I don't know if the
>stop position is optimum. I suspect not since it would have
>to vary with focal length.
>   Mensicus lenses have their principle points displaced
>toward the convex side of the lens, so, a meniscus with its
>convex side facing the film is slightly retrofocus and one
>with its convex side facing the object is slightly
>telephoto. So, the required bellows draw can be
>significantly less for a single cell facing the "wrong" way.
>Since the coverage angle of a long lens is limited the
>difference in aberrations for the lens in this position may
>be negligible. It may make a lens usable where its bellows
>requirement is right at the limit of the camera.
>   The difference is not enough to change the "look" and may
>not be easily noticable.
>   Where one has a triple convertible, that is two or more
>cells of different focal length, the longer FL should be
>used in front. I actually havn't checked this for bellows
>draw but there may be some, I think here probably not of
>practical interest.
snip

Richard,

Jim's photo is indeed excellent. It would be great to see the original trannie.

Thank you for the additional information - you have answered a question 
that was bothering me.

The Wollensak Raptar 13" convertible has one characteristic that is 
surprising given that the optimum position for single cells is behind the 
diaphragm. The barrel for the longer 25 1/2" FL front cell is chromed - not 
really what you want if the cell is placed behind the shutter. Your point 
about coverage for the longer FL may be the clue here - for 8x10 coverage, 
the area of the image circle that is compromised by any aberrations will 
not be used. The longer front cell of the Wollie is always left on the 
front of the shutter when using it alone. The telephoto effect results in 
less bellows draw, making it suitable for 8x10 bellows.

Cheers,
            Clive