Convertible lens question

[Scott Schroeder]

Excuse my lack of knowledge on the subject of lenses but I was hoping someone could enlighten me. I saw this lens
http://tinyurl.com/67274

and noticed it is convertible from 150 to 450 by removing the rear element. The convertibles I've seeen on the web seem to have different combinations that are put in front and rear to give different focal lengths. If this lens just needs the rear element taken off for a 3X focal length, what stops me from doing that with other lenses? I have the Rodenstock APO sironar-S 5..6/150. What would happen if I took the rear element off?
Just curious
Thanks

[Jim Galli]

New one on me. Usual convertible 150's like the older Symmar are 150 265. Go put your Sironar on your camera and give it a try. Come back here and tell the rest of us what you learned.

[John D Gerndt]

Lenses can be designed so that each componenet is usable as a lens in its own right. Many lenses are NOT designed this way. It makes sense you could squeeze more out of a lens design by committing each piece of glass to only one goal, one focal length and thus it is done now more often then once upon a time when 8x enlargements were less common. Still you can shoot a piece of film with half of any lens and see what you'll see.

Cheers!

[Victor Loverro]

All the convertible lenses I have heard of always place the single element behind the shutter. I may be wrong. Also, as Jim says, that does seem to be a stretch for the focal lengths. It is also advertised to sell "as is". Always suspicious when the numbers do not seem to fit.

[Mike Phifer]

I think the following gives some very good incite on the special requirements for a convertible lens, that John has previously mentioned.
Kingslake, in his "History of the Photographic Lens", 1989 Academic Press has a very good explanation in "Quadruple Cemented Lenses (pp 96-97)" on the Zeiss Double Anastigmat Series VII convertible lenses. He states,"Each half of the lens (he is talking about half of each individual 4 cemented lens cell) could be regarded as a new achromat located close to the stop, cemented to an old achromat on the outside, thus resembling the two components of the original Anastigmat arrangement (Zeiss’s lens that was free from astigmatism and had a flat field). The use of four elements cemented together permitted better correction of coma than would be possible in a triplet, and hence the rear of a quadruplet could be used alone as an anastigmatic landscape lens, which was not possible with the Dagor."
Essentually each cell is a fully corrected lens set in its own right. This is not generally the case of other lens designs.

I hope this helps. Mike