convertible lenses

[Dan Fromm]

Scott, the -S is a major redesign. There've been many discussions about performance differences between various versions of the lenses that have been engraved Symmar. To the extent that there's a consensus, it is that the gain from the jes' plain Symmar (6/4 plasmat type, sold as convertible) to the -S is large enough to make the -S preferable and that the gain from the -S to the APO- is minimal.

If you want a longer lens, just get one.

Its been a while since I've read this thread from the beginning and I don't recall whether Ole Tjugen has pointed out that all Symmars are double anastigmats and can be used converted whether Schneider says so or not. If he hasn't, he should have.

[Ole Tjugen]

I haven't pointed that out - since that would also necessitate the usual caveats about performance as converted lenses. the short version is that some "double anastigmats" are better than others for conversions, and that there are some simple rules, and that there are exceptions to every rule. Oh - and that many double anastigmats, when converted, perform somewhat less well than a converted Rapid Rectilinear. Which leads to the statement that the best Aplanats (AKA Rapid Rectilinears, according to "common knowledge") are actually sharper in the center of the field than all but a very few anastigmats. Which opens up a whole new can of worms, and starts a debate that won't be over until easter.

But for simplicity's sake, this picture was shot with a 150/265mm convertible Schneider Symmar, with the front cell removed.

[BennehBoy]

So, are the APO lenses convertible?

[Ole Tjugen]

The rear cell of all double anastigmats will form an image; whether or not that image is of sufficient quality to qualify as "usable" is a subjective call.

[Mark Sawyer]

So, are the APO lenses convertible?

Depends on the apo lens. "Apo" just means apochromatic, a higher degree of color correction. Since tessars aren't convertible, apo-tessars won't be. Since plasmats are convertible, , apo-plasmats will be. With all the caveats Ole just pointed out.

[BennehBoy]

Well it's a 150 APO Symmar, I guess I can just remove the front element and try it. I have a 300/5.6 but it's VERY heavy, and presumably also convertible since it's also a symmar?

[Bob Salomon]

Well it's a 150 APO Symmar, I guess I can just remove the front element and try it. I have a 300/5.6 but it's VERY heavy, and presumably also convertible since it's also a symmar?

So are all old Sironar lenses. But Rodenstock recommended removing the rear cell to convert. This would also protect the shutter.

[Mark Sawyer]

So are all old Sironar lenses. But Rodenstock recommended removing the rear cell to convert. This would also protect the shutter.

Hmmm, isn't it preferable to have the aperture in front of a single-cell lens rather than behind it? This is how Rodenstock built the Imagon...

[Dan Fromm]

Mark, the general rule is that the manfacturer knows best. And yes, the other general rule is that single cells perform a little better with the diaphragm in front.

Cheers,

Dan

[Mark Sawyer]

Mark, the general rule is that the manfacturer knows best. And yes, the other general rule is that single cells perform a little better with the diaphragm in front.

Cheers,

Dan

The single cells performing better with the aperture up front complies with the laws of physics. The manufacturers knowing best complies with the manufacturers' law of manufacturers knowing best. And manufacturers aren't stupid like me...

Who to believe? I'd like to hear Rodenstock's rationale. Maybe it doesn't really make much difference with some lenses, so it's better to protect the shutter. No judgement here, just wondering...

Mind you, if it's not symmetrical, it could be a triple convertible, so it would make a difference in the focal length. But even there, it's generally accepted that the front cell is put behind the aperture.

Cheers, Dan!