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

[Jim Hemenway]

Thanks for the additional information Richard.  I haven't noticed any 
focus shift at either the 13 1/4 or the 23 1/4 focal lengths.  That is, 
if I understand the concept of focus shift correctly, I can focus wide 
open, close the lens down to as much as f45, and with a loupe see that 
previously focused elements have not gone out of focus.

Jim - http://www.hemenway.com


Richard Knoppow wrote:
> 
>   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.
>   Paul Rudolph designed the Protar sections especially to be
> usable alone with good image quality. They are corrected for
>  coma and evidently pretty well corrected for lateral color.
> In a symmetrical lens, like the Dagor, correction for coma
> and lateral color is automatic because of the symmetry.
> While Dagors were patented and sold as convertibles the
> image quality is not very good unless stopped down to f/45
> or smaller. There is probably not a great difference between
> _complete_ Protars and Dagors, although it seems to me from
> my lenses that the Protar may have a little less zonal
> spherical (focus shift). However, this is a fault inherent
> in thick meniscus lenses so neither can be free of it.
>   A friend who is an actual lens designer ran the revised
> Protar design in a computer lens design program and found
> the program could not significantly improve it even with
> changes of glass type. Pretty remarkable.
>