Creating softness by unscrewing a lens element

[Tim Meisburger]

I understand that some of the soft focus lenses, particularly petzvals, created softness by shifting one element some distance from the other. Does that mean that I can add softness to a portrait by just unscrewing one element? Does this work for all lenses or only certain types (like petzval)? Does it matter if it is th efront element or the rear element?

I think I will give this a try, but was curious if anyone had any information.

Best, Tim

[Mark Sawyer]

The Tessar softens by moving the front element forward... or the other three backward, I suppose...

Cooke triplet portrait lenses and the Universal Heliar moved a middle element.

[Tim Meisburger]

Cool. Thanks Mark. I don't have any tessars, but I have an old plate camera that uses a helostar doppel-astigmat. Perhaps that is similar and will work. I'll give it a try if I can get it off the camera.

[Peter K]

The Tessar softens by moving the front element forward... or the other three backward, I suppose...

But if the front-lens alone is movable, such a lens was often used to focus on box-cameras.

[Steven Tribe]

F6.3 or F4.5? It is probably a dialyt (1air1 + 1air1) which is quite a good candidate for softness experiments.

[Tim Meisburger]

Well, I got the doppel-astigmat off the old Agfa folder (which is really lovely, by the way, with shift and rise like a Bergheil) and cleaned it up. (Stephen, it is the f4.5) The rear element was covered in white crud, and after I cleaned it I discovered that the inner surface of the double element (ie. the part I cannot reach) was badly affected by fungus, so that it looks unusable. I'll try to find another lens for the little 6.5x9 folder (perhaps I can find someone with good cells and a bad shutter), but in the meantime I took apart my other old folder, which is a no name 9x12.

The lens is also not a tessar. Its a Rodenstock Extra Rapid Aplanat f7.7 135mm. Do you think that might work? I'll knock up a lensboard tomorrow and give it a try. I also have some older lenses that originally came off a Crown Graphic (one of them is a Wollensak 127mm Raptar, the other is 135mm something).

Thanks for your help.

[Mark Sawyer]

But if the front-lens alone is movable, such a lens was often used to focus on box-cameras.

I'm surprised by that, but I'm fairly ignorant about the old smaller formats. Still, it seems the same physics would apply. Are you sure they weren't moving the whole lens?

Here's a thread on moving the front element of the old Wollensak Velostigmat Series II Tessar lens:

http://www.largeformatphotography.in...ad.php?t=57385

[Dan Fromm]

Mark, front cell focusing is fairly common in folding roll film cameras. That's front cell, i.e., all of the glass in front of the diaphragm moves as a unit. Read about in many of the "optics for photographers" texts. I just looked in S. F. Ray's Photographic Optics, 3d Ed, and I'm sure that Cox mentions it too.

In general, changing cell spacing changes focal length. Changing the focal length without changing flange-to-film distance changes the focused distance. Its a cheap expedient.

Many roll film folders use front cell focusing. I have a couple of Ensigns, including an 820 with an impressive looking 105/3.8 Xpres, and a Perkeo II with an 80/3.5 Color-Skopar that have front cell focusing. Folding Retinas, though, have unit focusing.

The pictures in the link you posted show that the Velostigmat Ser. II moves just the front element to reduce sharpness. I wonder whether refocusing brings the sharpness back, doubt that it will. Since you have the lens, would you try the experiment?

Cheers,

Dan

[Peter K]

I'm surprised by that, but I'm fairly ignorant about the old smaller formats. Still, it seems the same physics would apply. Are you sure they weren't moving the whole lens?

Frontlens-focussing was suggested first by H. D. Taylor, the inventor of the triplet, in 1899. But practically first used by Zeiss in the Twenties also with triplets and triplet-variations like the Tessar.

To focus a lens by frontlens-focussing one needs only a quarter of movement compared with moving the whole lens.

Of course this has also some drawback like changing the focal-lenght of the lens, movement of the rear and front nodals points etc. So this kind of focussing cannot be used for focussing from infinity up to close-up distances. But it can be used from infinity up to say 4 ft. without loss of image quality.

In a Schneider brochure from the Thirties lenses like the Xenar f/2.9 up to f/5.5, the Radionar a triplet where aviable either in barrel-mount or in frontlens-focussing mount. But a more complex lens like the Xenon was aviable in helical-focussing mount, and other lenses linke the (old) Symmar, the Angulon, the Isconar only in barrel-mount. And of course also in shutters.

Peter

[Mark Sawyer]

Mark, front cell focusing is fairly common in folding roll film cameras. That's front cell, i.e., all of the glass in front of the diaphragm moves as a unit. Read about in many of the "optics for photographers" texts. I just looked in S. F. Ray's Photographic Optics, 3d Ed, and I'm sure that Cox mentions it too.

In general, changing cell spacing changes focal length. Changing the focal length without changing flange-to-film distance changes the focused distance. Its a cheap expedient.

Many roll film folders use front cell focusing. I have a couple of Ensigns, including an 820 with an impressive looking 105/3.8 Xpres, and a Perkeo II with an 80/3.5 Color-Skopar that have front cell focusing. Folding Retinas, though, have unit focusing.

The pictures in the link you posted show that the Velostigmat Ser. II moves just the front element to reduce sharpness. I wonder whether refocusing brings the sharpness back, doubt that it will. Since you have the lens, would you try the experiment?

Cheers,

Dan

Thanks, Dan! I knew some medium and small format lenses changed focal length by moving internal elements; I just never knew the smaller Tessars were doing it by moving the front elements.

The Velostigmats I've worked with (12" and 15.5") pick up a great deal of spherical aberration as the front element is moved forwards. The focal lengths shift enoughthat you must refocus or you just get out-of-focus. I refocus to as sharp as I can get it wherever I have the front element set, and at the softest setting, there's still a distinct sharp image under the aberration-induced softness. (You can see this in a detail I posted in entry #17 in that thread.)

There was actually quite a bit of confusion a few years ago about how to use the Velostigmat's dial-in diffussion, as it threw the image out-of-focus, but when you refocused, it seemed as sharp as ever.

I'm surprised the shorter Tessars don't pick up the same aberrations when their front element is moved, but I'm hardly an optics expert. I just trudge along trying to pick things up by reading and experimenting on my own. Thanks again for helping me pick up a little more!