Metrogon cell spacing revisited HELP

OK - I sure don't see anything that looks like a flat plate of glass!
All 5 have curvature.

What does the 80 mm distance represent? The preceding distances look
like spacings, not cumulative distance. The only thing I can envision
80 mm meaning would be overall length?

Murray

OK - I sure don't see anything that looks like a flat plate of glass!
All 5 have curvature.

What does the 80 mm distance represent? The preceding distances look
like spacings, not cumulative distance. The only thing I can envision
80 mm meaning would be overall length?

Murray

I posted a zipped file containing the Zeiss, B&L and a third English
(apparently never commercialized) variant in the same family (Double
Gauss?).

Also, I posted a B&L lens factory data sheet for a 12" Metrogon. Kind
of disappointing the resolution is 'only' 32 (I assume lp/mm) on axis,
but maybe that's pretty good for a lens that covered 9"x18".

www.uptowngallery.org/Murray/Metrogon/

Looks like the 3rd lens was by Hasselkus & Richmond in England. It used
5 elements like the Metrogon, with the pair of them using the 5th
element on opposite sides of the assembly (one placed it in front, the
other in the rear). The English design claimed usability at f/5.6.

The Zeiss design claims 100 degree coverage at f/6.3, so maybe this one
will really cover 8x10, perhaps with some margin.

Murray

In article .com,
murrayatuptowngallery wrote:
Looks like the 3rd lens was by Hasselkus & Richmond in England. It used
5 elements like the Metrogon, with the pair of them using the 5th
element on opposite sides of the assembly (one placed it in front, the
other in the rear). The English design claimed usability at f/5.6.

The Zeiss design claims 100 degree coverage at f/6.3, so maybe this one
will really cover 8x10, perhaps with some margin.

Another similar lens is the Rectagon/Super Rectagon from Goerz. The
basic Rectagon must _just barely_ cover the image area of the 9"
square aero format, because it does *not* cover 8x10 out to the corners;
it comes up about 5mm short. I have tested this myself on an 11x14
camera with two different Rectagons so I am sure it is the case.

Goerz marketed the Super Rectagon for pictoral use on 8x10 cameras,
however, so it must cover. I don't know how it differs from the
standard Rectagon in design.

--
Thor Lancelot Simon

"The inconsistency is startling, though admittedly, if consistency is to be
abandoned or transcended, there is no problem." - Noam Chomsky

In article .com,
murrayatuptowngallery wrote:
Looks like the 3rd lens was by Hasselkus & Richmond in England. It used
5 elements like the Metrogon, with the pair of them using the 5th
element on opposite sides of the assembly (one placed it in front, the
other in the rear). The English design claimed usability at f/5.6.

The Zeiss design claims 100 degree coverage at f/6.3, so maybe this one
will really cover 8x10, perhaps with some margin.

Another similar lens is the Rectagon/Super Rectagon from Goerz. The
basic Rectagon must _just barely_ cover the image area of the 9"
square aero format, because it does *not* cover 8x10 out to the corners;
it comes up about 5mm short. I have tested this myself on an 11x14
camera with two different Rectagons so I am sure it is the case.

Goerz marketed the Super Rectagon for pictoral use on 8x10 cameras,
however, so it must cover. I don't know how it differs from the
standard Rectagon in design.

--
Thor Lancelot Simon

"The inconsistency is startling, though admittedly, if consistency is to be
abandoned or transcended, there is no problem." - Noam Chomsky

Nor do I. The standard Rectagon is a 4 element double Gauss lens
similar to the type used for the Kodak Wide Field Ektar.

Richard Knoppow

Is this in reply to the stuff I sent? The spacings in lens
prescriptions are from the center of one surface to the next. Overall
length of the lens is gotten by adding all the thicknesses and spacings
on the axis of the lens.
I am looking at the group using Google so there may be a missing
message.

Richard Knopow

murrayatuptowngallery wrote:
snip

Street wisdom on Metrogon cell spacing for barrel or other use is as
follows.


I would ignore street wisdom, and would only use patents as a very
rough guide. I designed and built a Topogon type lens about 15 years
ago (250mm EFL for 11x14) for my personal use, and despite the fact
that I knew *everything* about the lens including the as-built
parameters I still resorted to trial and error to get the central air
gap exactly right.

The following test takes advantage of the fact that a well-built
Topogon lens has an extremely flat field with virtually zero
astigmatism. Get the central air gap wrong and you will get lots of
astigmatism.

Screw the cells into the shutter or barrel and mount onto your camera.
Stop down the lens to about f/22 - f/32 to completely eliminate oblique
spherical aberration. At small apertures the main effect of getting
the central air gap wrong will be astigmatism, which will look alot
like field curvature. If you try this test at wide apertures you will
be completely confused by the enormous oblique spherical that occurs
off-axis. It may also be worth putting a green filter on the lens
since Topogons tend to suffer from chromatic variation of astigmatism.
Mount the camera to a tripod, and try to place the center of rotation
as close to the entrance pupil of the lens as possible. Rotate the
camera on the tripod while observing the image of a distant point
source such as a streetlight. Change the air gap by screwing the front
cell in or out until the point image stays sharp during the camera
rotation.

After you get the spacing figured out, you will probably need to
machine a spacer so that everything comes out right when you tighten
the cells.

As I said, I knew everything about my Topogon type lens: radii,
indices, spacings. Despite this, my "dead reckoning" of the central
air gap by re-fitting the design according to measured parameter data
was off by several tenths of a millimeter.

Brian
www.caldwellphotographic.com

Dr BC, good to hear from you again too. Your web site was sluggish or
down so I had to wait for a posting from you.

I acquired a complete 12" Metrogon pair, but it's not my nature to
allow the orphan front cell with apparent fungal etching (I previously
emailed you about) to rest in peace...it still passes light, so I'm
going to do something with it!

Murray