Convertible / part lens for longer focal length

[peter brooks]

I'm aware that some lens designs can be used with elements or groups removed to give longer focal lengths, with a corresponding change in aperture.

Convertible lenses are obviously designed to do this, my question is:

What other type (design) of lenses are particularly suited to this, and what is folks experience (and hopefully success) in doing this?

I've got a broad photographic interest, things don't always have to be razor sharp and I'm not adverse to softer focus, paper negs and thus longer 'top hat' exposure times with barrel lenses etc.

Focal lengths of say 18" / 450mm up - anything to not attach my barrel 480mm Apo-Ronar to the Canham :)

(The obvious answer is 'try it and see' but it would be great to learn from others experience first).

[Oslolens]

This is could be anastigmat, I got a 12" Manhattan where each lens is 24". Make one yourself with two +1 diopter to make a 500mm. The higher quality, the better result. See my flickr for simple bokeh test of lenses https://www.flickr.com/photos/47052585@N06/albums/72157634939838558

Sent fra min SM-G975F via Tapatalk

[Dan Fromm]

What other type (design) of lenses are particularly suited to this, and what is folks experience (and hopefully success) in doing this?

Old lenses engraved "Doppel Anastigmat" or "Double Anastigmat." Any more-or-less symmetrical lens. Dagor types, dialytes, double Gauss, plasmats are all worth trying. Depending on the type and design, a single cell's focal length will be 1.5 - 2x the complete lens' focal length. You'l get a long focal length lens, extension at infinity ~ focal length.

[Jim Noel]

This is an interest of mine. Every lens that comes into my possession is subjected to a trial focusing of each element. I have on occasion also separated elements into individual components. Many interesting images have resulted.

[Kevin Crisp]

I've used single elements of Protars and convertible Symmars. Also I have a convertible rapid rectilinear. All of them have produced decently sharp and contrasty negatives. You have to check and adjust focus when stopped down, though. Many people declare the single elements worthless but I suspect they don't adjust for focus shift. The very common single element protars (19" aka 18 7/8th") would be a lens in the range you're looking for.

[Dugan]

I have the front group of a 10" f8 Goerz Gotar, gives 349mm f12. I haven't film tested it yet.

[peter brooks]

Thanks everyone for the replies, suggestions and advice. It makes sense that any virtually symmetrical lens could be used... I now have a whole load of focal lengths that I never knew I had!

Now, where's that tiny 203mm Ektar...