Graflex.org

[primus96]

I have just won one of these lenses from you-know-where.
It is a 130mm f6.8 in a dial set Compur shutter. Any idea of coverage? I know the lens is reckoned to be from a Goerz Tenax 9x12, the shutter says 'CP GOERZ' on the dial.
Im hoping it'll be enough to cover 4x5.

[primus96]

I did start another topic on a Goerz Doppel Anastigmat. I made the previous post when I realised I had stated its focal length incorrectly.

[Dave]

Same answer as t.r. gave before: with a 13cm Dagor on a Crown, you'll run out of camera movements before you run out of image circle.

[t.r.sanford]

The Goerz "Doppel-Anastigmat" (or "Dagor") was designed to cover a 70-degree field. This means that a 130mm. focal length should produce an image circle just over 180mm. in diameter, which is more than enough to cover a 4x5-inch frame, and allow some perspective control movements.

[primus96]

Does it necessarily follow that every lens by CP Goerz entitled 'Doppel Anastigmat' is of the design later termed 'Dagor'.
The term sounds a little generic to me.
Is there anything else they made that is a double anastigmat?

[Dave]

You're right that "Doppel Anastigmat" is a generic term. Schneider's Doppel Anastigmat became the Symmar-- you can see a 1930s-vintage "Doppel Anastigmat Symmar" sold with this fairly disgusting Linhof Technika I recently, on your favorite auction site . . .
http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=3806018963&category=11717

A Dagor (six elements in two groups, 3 3) and a Symmar/Plasmat (six elements in four groups 2 1 1 2) are different lenses. What they have in common is they're more-or-less symmetrical, and made of two independent lenses mounted backwards to each other-- which I guess explains the 'Double Anastigmat".

I'm almost certain the only thing Goerz made called "Doppel Anastigmat" was the Dagor and its precursors. Did they ever make a plasmat design?

And another thing . . . the Linhof practically had flies buzzing around it. For $405, the buyer could have got a damn good Super.

[t.r.sanford]

It would be interesting to find out whether C.P. Goerz coined the tradename "Dagor" because other makers were producing "double anastigmats" and they wanted their original to stand out from the crowd. I vaguely recall seeing a "Doppel-Anastigmat" from Friedrich or somebody.

There evidently was a "double Protar" from Zeiss that consisted of four cemented elements, all arranged on one side of the stop. The account I read of this explained that two of these could be arranged symmetrically, one on either side of the stop; one group then could be used alone, or both together. If you did that, you'd have something that justly could be called a "double anastigmat"...

It's not surprising that Goerz wanted a proprietary name for its design.

I never heard of Goerz making a "Doppen-Anastigmat" other than the "Dagor."

[primus96]

The camera looks ropey and from my point of view the lenses all look as if they need major attention.
That old Symmar looked defective too.

However it may be faulty comprehension on my part but I thought that Carl Meyer took the symmetrical design of the Dagor as his cue when designing the Plasmat. There are convertible Plasmats just as there are Symmars for Linhof's which are convertible.

Does the Symmar suffer the phenomenon of focus shift or does that design elininate it?
I wondered how pronounced the effect is going to be on my 130mm f6.8 Dagor.
At f16 or 22 the gg screen image is non too bright, even when I throw my coat over the camera to sheild the screen.

[t.r.sanford]

In the old days, people dealt with determining sharp focus when stopped down to f:22 or thereabouts by drawing an X in the center of the frosted side of the groundglass, then adhering a circular microscope cover slip over it with Canada balsam. This allows you to see the aerial image, much brighter; and you determine sharp focus by moving your head slightly from side to side. If the X moves across the image, the lens is not in focus; if it does not, you've found the focal point.

The direction of the apparent movement indicates whether the lens has been racked in too close or not close enough; I forget which is which. I did this a long, long time ago with a beat-up old miniature "Speed Graphic," and it does work. It also impeded my ability to compose on the groundglass, so I never repeated the exercise.

My impression is that Dr. Rudolf, the inventor of the "Protar" and the "Tessar," designed the "Plasmat" for Meyer after leaving Zeiss. Until fairly recently, I think, "Plasmat" was a proper name, not a description of a lens's construction; but these things change over time.

[Dave]

[t.r.sanford]

I have a vague recollection that the first-generation anastigmats achieved their flatness of field over wide angles of coverage, and their correction for astigmatism, at the expense of undercorrected spherical aberration. This would tend to produce an apparent focus shift as you stopped down, because you'd be blocking the rays passing through the edge of the lens.

I've not used a "Dagor" either, but I know the focus shift was generally said to be a characteristic of the type. It didn't bother people who liked the lens, as a great many people did.

It would be less pronounced with shorter focal lengths, so might not be discernible in a 130mm. lens.

I like the idea of the laser pointer.

[primus96]

Is the phenomenon of focus shift really going to affect only the longer focal length Dagor?
I think that a Dagor suffers from sperical aberation & thats a change of focal length towards the edge of the lens. Is this what they mean?

I have only taken a few sheets of Fuji NPS 160 so far, incl some flowers with the drps of a recent rain storm on them. I am waiting to see how the print turns out.

So far the eges arent that bad, even at f8, which is only a half stop down from wide open.
I am going to have to do a comparison with my Ektar or Xenar and shoot B&W this time.

[t.r.sanford]

The apparent focus shift would occur with a lens of any focal length.

However, it would be proportional to that focal length, and therefore less with a lens of shorter F.L. If it were less than the depth of focus at the taking aperture, you probably wouldn't notice it.

If it became a problem with critical focusing, you could deal with it as our pioneering ancestors did, by using a magnifier and refocusing at the taking aperture, after composing with the iris wide open.

Too much can be made of lens aberrations. The predecessor to the first-generation anastigmats, the "Aplanat" or "Rapid Rectilinear," also suffered from undercorrected spherical aberration, and astigmatism as well. At f:16 or so, you didn't notice. Edward Weston used one at f:64, and no one ever complained that the photos were unsharp...

I'd expect your "Doppel Anastigmat" to work well right out to the corners of the 4x5 frame. These lenses remained popular for well over half a century because of their excellent covering power and contrast.

[primus96]

I would love it if anyone can tell me what direction the focus shift tends to go?
If you focused with the loupe around the centre of the image would this still be sharp when stopped down to whatever the taking aperture?

The edges aren't often that important in a image, except if taking a landscape.

[t.r.sanford]

Never having worked intensely enough with an early anastigmat to experience "focus shift," I only know what I've heard about it, and what I can surmise.

I don't know the direction in which the focal point moves as you stop down -- my guess is that it would move back rather than forward, but I wouldn't bet money on it. Whichever way it moves, the phenomenon would affect the center too. This is why our forefathers focused at the taking aperture, when using early anastigmats.

You might try moving the lens out to double extension, and arranging a small filament lamp (like a flashlight bulb) between ten and eleven inches from the lens. If you turn the lamp so the filament is parallel to the focal plane, you'll have a flat focusing target that you can see at any aperture.

Focus critically on it with the lens wide open, using a magnifier, and make some kind of witness mark on the track and the bed (two pieces of white tape and a fine pen charged with India ink might be a convenient way to do this). Then stop all the way down and focus critically again. See whether the two ends of the witness mark still line up.

If they don't, you've identified focus shift. If they do, you haven't got a problem.