[LargeFormat] Some simple lens coverage tests with paper negatives

[Jim Hemenway]

Hi Richard:

Thanks, very much, for the information.
-- 

Jim - http://www.hemenway.com



Richard Knoppow wrote:

<SNIP>
> >
>   The Velostigmat is essentially a copy of the Protar. It
> was also sold under the Raptar name and a great many were
> made for the military to meet the Protar/Dagor
> specification. They are actually very good lenses.
>   The two cells of the B&L Protar are actually the same
> focal length. What you are seeing is the difference in the
> locations of the principal points. For a single meniscus
> lens one of the principal points lies about at the apex of
> the convex side of the lens and the other in space in front
> of the apex. So, when the lens is used in its "normal"
> position i.e., behind the stop with the concave side facing
> the film, it is slightly retrofocus. When used in front of
> the stop, with the concave side facing the object, it
> becomes slightly telephoto. There is a significant
> difference in the bellows draw although the focal length and
> image size will be the same. This is true of all meniscus
> lenses from the simple single element ones on box cameras to
> Dagors and Protars.
>   The easiest way to find the true focal length of a lens is
> to focus it first at infinity and mark the bellows draw. The
> focus it for exactly 1:1 image to object size and measure
> the difference in distance from the image. This is exactly
> the focal length. As a check, the distance from image to
> object at 1:1 is exactly four times the focal length.
>   A good way of finding infinity focus without a
> sufficiently distant object is to autocollimate the lens
> using a mirror. Ideally, this mirror should of the first
> surface type but an ordinary shaving mirror will work, just
> make sure the plane side is used, not the magnifying side.
>   Place the mirror in front of the lens, the distance does
> not matter, and put something like a pencil flashlight
> against the ground glass. You an also mark a cross on the
> ground glass near, but not exactly at, the center.
> Illuminate it with the small light. The mirror will reflect
> an image of the cross back to the ground glass. When its
> sharply focused the lens is exactly at infinity focus.
>   If you know the focal length you can measure one focal
> length from the image toward the lens. That will be the
> second, or rear, principal point. When the lens is turned
> around and focused again the measurement will indicate the
> location of the first, or front, principle point. Knowing
> the locations of the principal points is sometimes useful.
>   If you autocollimate a large (in diameter) lens with a
> target at the focal plane you can adjust other optical
> devises using it as a collimator. The target will appear to
> be at infinity.
> 
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@ix.netcom.com
> 
> _______________________________________________
> LargeFormat mailing list
> LargeFormat@f32.net
> http://www.pairlist.net/mailman/listinfo/largeformat