Convertibles: Why use the rear cell only?

[Paul Schilliger]

There is probably a reason why the convertibles should be used with the rear component when used as such. But there is a decisive advantage at using the front cell: the bellows extension is 25% shorter (acts like a tele lens). However, from examining two images taken one with the front cell, the other with the back cell of a G-Claron 305, it looks as there might be some haloing around the highlights on the one taken with the front cell. But I'm not certain this is from using the front cell, could someone confirm this? And what about screwing a UV filter into the missing element tread? Would it greatly affect the optical performance?

BTW, some lenses are not given as such but make excellent convertibles. I'm thinking of the Fujinon A. The 240 produces a good 420mm lens.

[Bob Salomon]

"Another essential advantage of Sironar-type lenses is that their special optical design allows the FRONT COMPONENT alone to be used as a "telephoto lens".

From the 8/75 Rodenstock brochure for Rodenstock Sironar lenses. There is another very important reason to use the front element rather then the rear. The front element will protect the aperture and shutter blades from damage while the lens is on the camera.

use the rear element only and the shutter blades are very easily damaged.

[Paul Schilliger]

The Sironar design is clever and certainly an improvement over other convertibles. Blades damage and risks of dust into the lens are the reason I would seal the opening with a UV filter. But from it's place where the rays are converging, I suppose that it should be made of very high quality optical glass and even then, I'm not sure it would not affect the image. Has anyone tried?

[Bill_1856]

The green "2nd aperture scale" for a Schneider Symmar is based on use of the rear element.

[Bob Salomon]

"ased on use of the rear element"

Just out of curiosity what would be the difference in exposure if you used the green scale with the front group?

[N Dhananjay]

The position of the stop is important in controlling certain abberrations with a meniscus lens. Also, the barrel distortion from the stop in front of the lens is considered preferable to the pincushion distortion from the stop behind the lens. But it should be kept in mind that using a single element anyway upsets the corrections. Also, as pointed out, the nodal point of the lens actually lies outside the surface of the glass, resulting in longer bellows extension when using the element behind the stop and shorter bellows length when using the element in front of the stop. Also, using the cell in front protects the shutter mechanism, although whether exposing the shutter is a problem probably depends on your working conditions.

Most convertible lenses (but not all) have some power shifted to the front cell to optimise them somewhat better for infinity - the Schneider convertible Symmars, for example. Using the front cell will give you a somewhat longer focal length (so compensate the f stops accordingly), but keep in mind that it is the rear cell that is optimised for use singly. The performance of the front cell (on the one sample I have used) is condierably worse - lots of spherical abberrations, haloing etc.

Cheers, DJ

[Paul Schilliger]

So there is an issue with image quality when the wrong cell is used. Does anyone have experience with the Symmar-S line or the Sironars (non Apo) produced after the convertibles era, when used with a single cell?

[Glenn Wishon]

Front cells have one type of correction and two or three types of distortion. Rear cells have spherical aberration and coma, but unlike the front cells, these distortions can largely be corrected with a yellow or orange filter. Whenever you use just the rear cell, use an 11 or greater filter--either push on or screw on, behind the rear element. If you use just the front element, there is no such correction available, and the best you can do is use the mid-range of aperature, avoiding the largest and the smallest stops.

[Mark Sampson]

My experience with a 180/315mm Symmar is that the front cell used alone gave results sharp only in the center- very soft by the edge of the field. Good for portraits maybe. That was years before I knew to use the back element. When I tried photographing with the rear cell, the quality was much better, I would say usable; but not nearly as good as my 300mm Nikkor-M. I never made enlargements from the comparison negs, maybe it's time to do that. ...Somewhere at work I have the brochure (from around 1972) introducing the Schneider Symmar-S lenses, where they mention that they eliminated the convertible feature in order to improve performance at the designed focal length.