[LargeFormat] Some simple lens coverage tests with paper negatives

[Richard Knoppow]

----- Original Message -----
From: "Jim Hemenway" <Jim@hemenway.com>
To: <largeformat@f32.net>
Sent: Saturday, March 22, 2003 8:09 PM
Subject: Re: [LargeFormat] Some simple lens coverage tests
with paper negatives


> Hi Richard:
>
> Thanks for noticing my poor attempt to catalog my lenses
for use.
>
> The front cell is marked as follows:
>
> PROTAR VII Focus 23 1/4 In.   BAUSCH & LOMB Optical CO.
ROCHESTER NY.
> U.S.A.  No 3217344
>
> The rear element is marked the same and is No 3217343
>
> When using both elements the approximate focal length is
340mm.
>
> I measured the focal length at approximately 590mm with
the rear cell
> behind the lens, and it's approximately 560 with the front
element on
> the front.
>
>
> I also have a very beat up 16 1/2 inch Velostigmat Triple
Convertible
> whose focal length is indeed 16 1/2 with both cells.  But,
when using
> either of the individual cells, either in front of or
behind the
> shutter, the results are very fuzzy on the groundglass
when focused as
> best as I can... looks somewhat like the corners of the
paper negatives
> from the APO Ronar CL 300mm.  The rear cell of this
Velostigmat is
> unmarked.
>
> I didn't take any shots with the 16 1/2 inch as it has
been stripped of
> a shutter and an iris.  All that remains is the carcass of
a Packard
> shutter.  I plan on eventually asking SK Grimes the cost
of putting the
> cells into a modern shutter.
>
> BTW, the Protorlinse (from the 8x10, the one that
vignettes on 11x14)
> )does not have matched cells, the printing style and the
general look of
> the rear 41cm cell makes me think that it is perhaps 20
years newer than
> the front 59cm cell.
>
> I'm very appreciative of any information which you can
provide about
> these lenses.
>
> After going through this bunch of lenses, and until I get
rich, I'm
> going to shoot 11x14 with the 360 Symmar-S... a wonderful
lens,  and the
> 420 APO-Ronar-CL at around f45 - f64... which should prove
okay until I
> get the Velostigmat mounted.  I also want to shoot a
chrome or two with
> the Protar VII at 340 and 590mm to see if I get any nice
results.
>
>
> Oh! One other thing if you have any info... the 210
Ultragon and the 210
> Gerogon look almost identical in my hands except the the
Gerogon has a 6
> blade iris and the Ultragon has 5 blades.  Do you happen
to know if they
> are both made by Rodenstock?
> --
>
> Jim - http://www.hemenway.com
>
>
  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