How Do I Use a Schneider Convertable Symmar?greenspun.com : LUSENET : Large format photography : One Thread
I recently purchased an older Schneider Convertable Symmar lens. It is marked 180/5.6 and 315/12. According the the serial number, it was made about 1971 and is all black.
The salesman told me to remove the rear component to use the lens at 315. Another question on this site also references removing the rear component. However, when I called Schneider to get a replacement index screw to engage the slot in my board (the original was missing and Schneider customer service was super!), I asked about the "conversion". They said to remove the front component, as does Ansel Adams in his book "The Camera".
So I tried both ways. With the rear removed I get an infinity focus at about 10.5" between the standards (a bit short of 315mm), and with the front removed I can't get anything to focus. That would appear to agree with the "remove the rear" recommendation. The complete lens focuses at infinity with about 7" between the standards (seems to agree with 180mm). Removing the rear component does require more effort, but it keeps the shutter protected from the elements and retains the filter ring---so I actually prefer that arrangement.
What's the correct method to convert a Symmar, and why do some sources say to remove the front component and others the rear? Were these lenses made in two different configurations?
-- Daryl George (email@example.com), May 18, 1998
Removing the front element is the correct configuration for the use of a convertible lens. It will take more bellows draw if the front element is removed as opposed to removing the rear. If you are racking the bellows all of the way out and still not getting focus at infinity, your bellows may be to short for a 315mm lens. The lens to film plane distance is measured from the rear of the lens with the front element removed. That is why prople try to remove the rear element, to make the bellows draw less. This can lead to a reduced focal length and some focusing problems.
To illustrate, I have a 150 convertible Symmar. At 150 mm the lens board to film plane distance is 5 1/2 inches, or about 150mm. Converted, by removing the front element. the lens board to film plane distance is 13 inches or 325 mm, approx. The distance from the back of the rear element to the film plane, however, is 11 inches, or close to 265 mm. The lens board to film plane distance is 7 inches or 175mm with the rear element removed, as opposed to the front element. These bellows measurements are based on focusing on an object at 25 feet, not, infinity.
You can see that the removal of the elemements doesnot result in the same focal length being produced. To address the filter concern, it is normally preferable to place the filter behind the lens, and to focus at the shooting aperature with the filter in place. The use of a filter, generally a yellow or orange is recommened to correct for the fact that by removing a lens element the colors of light will not focus on the same plane.
I hope this helps address some of your concerns, I am not an expert by any means, but have used convertibles with some success. These observations have resulted from my reading and practical use of the lenses. Good luck.
-- Marv Thompson (firstname.lastname@example.org), May 18, 1998.
Thanks for the help, Marv. Following your advice, I retried removing the front component. As you suggested, I had to extend the bellows much more than I was expecting in order to achieve focus. I got an infinity focus with about 15 1/2 inches between the lens board and the film plane. Subtracting 2 inches for the rear component extending behing the board gives 13 1/2 inches, which is fairly close to the rated 315mm focal length. There was still enough bellows and rail left that I could get a close focus of about 15 feet. The field of view agreed well with a 35mm zoom lens set for 90mm (the 35mm equivalent). Now everything seems to fit.
You mentioned that some people do remove the rear element to reduce the bellows extension. When I tried that, it looked like I was getting the equivalent of about a 10 1/2 inch lens. That configuration might work as a portrait lens. A slightly soft image would be a problem. Do you have any information on what the f/stops would be with the real component removed?
-- Daryl George (email@example.com), May 21, 1998.
Strictly guessing, I would say to use the same f-stops as when the rear element is removed. I have tried shooting with the front element only on a 12" Turner and Reich Convertible Triplet, and used the 19" f-stops. It was a little soft, but the exposure was about right. Try experimenting a little, that's half the fun anyway!!
-- Marv Thompson (firstname.lastname@example.org), May 21, 1998.
In mine opinion the main advantage of removing the front lens is that in this way the diaphragm stays before the lens, so the light travel is: subject-diaphragm-lens-film. It seems that this arrangement corrects aberrations of the single element better than the other pathway (i.e.subject-lens-diaphragm-film).I know that the first lenses used in photography, very simple and poorly corrected for aberration, have been used with the the diaphragm in front of the glass. I tried both the ways with an old Zeiss Protar VII Convertible, using the short-focus group (ca.300 mm focus) placed behind the diaphragm and in front of it, and working in both cases with f/22 on a 13x18 cm film.. In the former arrangement, the picture resulted much more sharp in the borders, while the central parts of the images were in effect equally crisp. I would like to know more about the influence of the diaphragm position in a lens. Thanks, Franco.
-- Franco Rallo (email@example.com), December 02, 1998.
Yes. The correct method is to remove the front element on the Symmar lens. Not mentioned here is a term called focus shift. When using only the rear element the plane of focus will move as the diaphram is closed down. You should focus at the shooting aperture to get a reasonably sharp photo.
-- Kreig McBride (firstname.lastname@example.org), December 27, 1998.