Convertable Plasmat Set

By Jean-David Beymer for the Large Format Page

Ron Wisner does make a Convertable Plasmat Set. Actually, he makes three sets. One for 4x5, one for 5x7, and one for 8x10. The 4x5 set has 5 cells, two 250mm, and one each of 350mm, 400mm, and 450mm. Each cell, used alone, is f/13. For the 4x5 set, he has rebuilt a Copal #1 shutter so that any cell can be used either in front or behind the shutter, so you can mix and match, using any cell in front and any other cell behind. You can also use single cells. Using combinations results in shorter focal lengths than using pairs. Using a single cell does not give you complete correction for certain aberrations especially distortion, coma, and lateral color, so you would want to use a fairly strong monochromatic filter (or, more usually a strong yellow such as #12 or #15) to correct for lateral color. You can also order a "corrector" cell that has zero power, but corrects for these aberrations. I do not know when these will be shipped, however. Mine has been on order for several years. The Symmars are Plasmat design also. Many older convertable lenses have received a poor reputation because they were only 2 cells, so you necessarily got the 3 aberrations mentioned above, and people did not always know what to do about them. In the old days, when film was not panchromatic, the lateral color was less of a problem than it is now with more modern films.

I have not visited the Schneider factory to see if they are grinding the glass specially for Ron Wisner or not. Ron says they are custom ground for him and I have no reason to doubt him. If you glance through the lenses offered by Schneider, you will not find any with the apertures and focal lengths that are used, so I am more inclined than ever to believe Ron. Furthermore, if the glass were off-the-shelf stuff, it would not have required 2 years to get my set and over 3 1/2 years to not get my corrector lens.

This set of lenses is different from older convertible plasmat designs in various respects. You can buy them new and multicoated. They come in a modern shutter. They are easier to find than a comparable Protar set (and probably cheaper, too). Since modern glasses can be used in their design, they probably perform better than older convertible lenses. Remember that some convertable lense came with a total of 2 cells, giving only three focal lengths. Using single cells gives you residual aberrations. With the Wisner (and Protar) sets, you can get most of the focal lengths by using pairs of cells, so that these residual aberrations are corrected.

I have purchased Ron Wisner's Convertible Plasmat 4x5 lens set. It is physically beautiful, small, and works very well. The fancy wood box and engraved placque are very beautiful. I was afraid that, since the individual cells (usually used 2 at a time) are only f/13, it would be dim and difficult to focus. (When used two at a time, though, maximum apertures of f/9 can be obtained, depending on the focal length desired.) This has not turned out to be a problem.

My f/8 S.A. 90mm seems dimmer than the Wisner lenses. The problem is that with a WA lens, the light coming off the ground glass to the eye is going on a straight line from the diaphragm of the lens. Consequently, only a small proportion goes to the eye to be seen. With a longer lens, the angle is not so far off and the image appears brighter. Now a field lens, in particular a Fresnel lens between the eye and the ground glass, will fix this and I would never have mentioned it at all but for the fact that such a lens only works well when the camera lens is on axis. When it is off axis, things are worse than with no field lens at all. I actually took my Fresnel off the camera and keep in in a drawer at home. To see near the edges of the image, I must move my head so as to be peering along the line through the diaphragm of the lens. To use a magnifier is a real problem. I have a little Loupe I got from Edmund Scientific. It is about 9x. It is adjusted so that it focuses on the ground side of the ground glass when I am on the shiny side (i.e., when I am outside the camera). But for the WA lens, I must tip the loupe, and I cannot do that and keep it focused. In fact, I cannot do it at all unless I reverse it and try to adjust the angle of the loupe, the distance of the focus of the loupe, and the focus knob of the camera. So I swear a lot. Or, since I am nearsighted, peer over the top of my glasses and focus without the magnifier. Note that I said "appears dimmer." I know perfectly that it must be brighter. The lens is perfectly good and obeys the laws of physical and geometric optics.

Making them f/13 keeps them small and light, which is a great advantage. They are kind-of expensive, but when you figure you have about 8 different focal lengths, that is actually a pretty good deal. I sure would not want to carry 8 Apo-Symmars around in focal lengths up to 450mm, even if some rich uncle would buy them for me.

The 4x5 set makes images every bit as good as I get from my 210mm f/5.6 Caltar-S lens, my Nikor-m 300mm f.9, and my former 135mm f/5.6 Symmar-S. This judgement is by eyeball on 11x14" prints from 4x5 negatives. I have never troubled to make resolution chart tests, etc., for various reasons. I greatly appreciate their small size and light weight. I usually carry only this set and an S.A. 90mm f.8.

The only negative factor is that, since his stuff is so popular, and since he is such a perfectionist, delivery times are so long as to try my patience.

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