Graflex.org

[primus96]

I have shot some negatives with this lens.
I have focused very close up and close to infinity.
My experience seems to suggest that if I focus on a distant object and stop down the focal point shifts nearer when stopped down to f22. This is when focusing on part of the scene at the centre of the screen
The gg screen is so dull at f22 its virtually impossible to tell if the image is sharply focused or not, even with a loupe.

[primus96]

I have resorted to throwing my coat over the top of the camera to exclude as much light as possible.
I then shot different scenes where there was a distant electricity to focus on.
I took two exposures, one focused the normal way and another when stopped down.
Even the 130mm Dagor exhibits enough focus shift to be noticeable.

Would the shift be greater for a Dagor of longer focal length?

Next thing is to do a comparison with my 150 Xenar at the same aperture.

In my experiments I have used the Classic Pan 400. It looks to have more contrast than HP5 @ 400ASA.
Not sure about grain but a 40x50 enlargement would be necessary to show that and who cares when the stuff is so cheap?

[t.r.sanford]

Right; a good focusing cloth, and a magnifier with an opaque base that seats firmly on the groundglass, are very helpful if you want to focus at taking aperture. (I've used the coat trick from time to time and, in my opinion, a purpose-made focusing cloth is better!)

I'd expect the focus shift to be more or less proportional to the focal length of the lens, i.e. you might expect a longer lens to exhibit greater focus shift. What we need here is some input from people who have used "Dagors" for a long time, and who thus have become familiar with their characteristics.

As noted earlier, the days of small apertures and unbrightened groundglasses gave rise to an aerial image focusing trick. I tried it, decades ago, and it works. You draw an X on the matte side of the GG (side toward the lens), right in the middle, using a very fine ink pen. You then put a drop of Canada balsam on top of it and gently place a clean microscope cover-slip atop it (the usual microscopist's method is used to exclude bubbles).

What this does is get rid of the matte surface, so you see the aerial image; the cross is a reference point for your eye. To use the thing, you focus roughly, then move your head slightly from side to side. The cross will appear to move, relative to the image, when you are out of focus. When the focal point is right in the plane of the cross, it will not move relative to the image when you move your head, and you can expose with confidence

If I were planning to do a lot of work with a "Dagor" or a "Protar," I would make a GG like this, though I wouldn't want it to be my only focusing screen.

[primus96]

I have shot a fair bit of stuff with this lens. All b&w so far, FP4 and PL25.

Now I have mastered the focus shift but the edges of the image are very soft. This is perfecty visible on a 8x10 full frame print.
Is lens cell spacing really critical because i'm sure this lens could be better than this.

If it ain't worth throwing money at it'd make a a fine lens for a 2x3 graphic. Whoever used it before me had a 2x3 Busch Pressman C.

[t.r.sanford]

I'd imagine that the unsharpness at the corners becomes less objectionable as you stop down, but it certainly shouldn't be pronounced at f:11 or smaller. The elements may, indeed, be spaced wrongly, or decentered. A good lens repair facility could tell you, but these are becoming rather difficult to find.

[Henry]

Thanks, t.r., for this clear explanation of the aerial image method of focussing. Earlier posts under this thread left me puzzled. I see now that this is the same principle used by some devices to focus the projected image of an enlarger at the plane of the photo paper, where you're actually focussing on the grain of the negative. Takes me back to the old wet-darkroom days. (In the digital/pixel world, the rough equivalent in Photoshop seems to be the strangely-named "Unsharp mask" function.)

[ This Message was edited by: Henry on 2004-05-16 10:56 ]

[t.r.sanford]

Yes, enlarger magnifiers rely on your being able to focus on an aerial image when a reference mark is provided, the little crosshair reticle at the focal point of the magnifying instrument.

The method I am recalling did not depend on your ability to identify sharpest focus, even with the reference mark. It was designed for use with slow lenses stopped 'way down, where depth of focus is great. The system worked because of the distance along the optical axis between the true focal point and the reference mark in the focal plane. When you moved your head back and forth, if the two points were not coincident -- one was closer to you, or farther away -- you would see one appear to move across the other.

I have a handfull of old, small B&L "Protars" in barrel that I picked up thirty years ago, when no one knew what they were and the old-line camera stores were selling them for a few dollars apiece. One of these days, I mean to make an aerial-image focusing screen and give them a workout. A maximum aperture of f:18 does not conduce to groundglass focusing!

[primus96]

Would SK Grimes have the ability to do the work, as part of a overhaul on the shutter too?
Tis a shame, because where it is sharp its wonderful.

[t.r.sanford]

S.K Grimes would be my first choice for checking a lens mounted in a shutter. You might drop them a line and see what they have to say. I don't know how deeply they get into optical testing, but they ought to be able to determine the nature of the difficulty.

[primus96]

When I asked Grimes theysaid that if the lens focuses fine around the centre then there cant be any mis-spacing of the elements. The lens is very sharp around the centre but the edges and corners leave a lot to be desired.
If this 'Doppel Anastigmat' is indeed a Dagor, how can I tell by looking at the cells if I unscrew them?
I look down through each & see two reflections. Is this right ,cos I thought a dagor was two cemented triplets (6, 2). The internal surface of each cell appears to be concave. This doesnt help as the Goerz company did two double anastigmats. We know one as a Dagor, the other is the dialyte formation Goerz called a Celor.

[t.r.sanford]

The "Dagor" was, indeed, two cemented triplet groups. The diagrams I've seen show the inner faces of each group concave. The "Celor" (and, I think, the "Dogmar")used four airspaced elements.

It might be useful to know whether you can bring an off-axis image point to a sharp focus at a distance different from that required to focus sharply at the center, or whether the off-axis image is irremediably soft.

[Nick]

[Dan Fromm]

[primus96]

If anyone is interested in this beastie.
Its fine for 4x3 or 2x3, but stretched on 4x5.
Drop me a private message if interested.

[RichS]