Convertible lenses for ULF

[e]

Hi all,
I was wondering if anyone out there might be using their Computar/G-Claron/Dagor lenses as convertibles for ULF. Specifically the 240/270mm Computars and the 355 G-Claron and the older uncoated 300mm f6.8 Dagor. What were the results using the back lens elements only 7x17 and up? Thanks, Emile. www.deleon-ulf.com/

[N Dhananjay]

Your best bet for convertible lenses for ULF are probably long Protars and Dagors. I've seen double Protars of around 16" focal length which would convert to about 28" with the fron element removed. Protars are probably preferable to Dagors for use as convertibles, since an extra element was used in each cell to correct each cell for coma. But Dagors certainly will work very well at small stops and provide slightly more coverage and might be more easy to find. Cheers, DJ

[CP Goerz]

The extra element in Protars was put there since they couldn't make a three element design since Goerz already had the patent...Zeiss had the chance to own the best lens design in history.... but turned it down. Any Dagor is convertible and the focal length of a single cell is 1 3/4 X the focal length of the combined lens. The aperture will then be raised to F13.5 approx. As you are probably familiar with the Ansel shot of 'Frozen Lake and Cliffs' image it was shot with a single cell of a Dagor.

Some early Dagors have two aperture scales for the combined and single cell. The usual rules apply with the single cell, focus slightly stopped down and use a #15 or 22 filter to prevent colour fringing and to increase the contrast which will drop slightly. Focus shift I've found to be minimal at most, the depth of field always seemed to take care of any problems encountered.

A better choice for ULF lenses is the normal array of process lenses seen on Ebay and elsewhere. The Goerz Artar design is one you should look out for(other variations include the APO Ronar, APO Nikkor, G Claron etc).

CP Goerz

[CP Goerz]

PS:The G Claron isn't the same design as the Artar/Ronar etc but is in the process lens realm.

CP Goerz

[Struan Gray]

You might also consider the old 360 mm Symmar convertibles. They cover 500 mm at 360 f5.6 and at least that when converted to 620 f12. I have only looked at the ground glass image on my 12x15 camera, but I'd be happy contact printing either focal length at normal taking apertures. Big and heavy, but a lovely chunk of glass and the Compound shutters are reliable. There is also a later 355 version in a Copal 3.

[sanking]

I used a 360mm Symmaer convertible on the 7X17" format for several years and it gave very good results with the combined elements. However, using either of the cells individually resulted in awful performance at the edges of the format. Performance would probably have been adequate for 8X10 but for 7X17 I found it to be awful, and I always stopped down to f/45 or f/64 when using the cells by themselves. I would estimate that when using the cells individually the acceptable angle of coverage is less than 40 degree.

Just to be fair, I have had pretty much the same experience with the individual cells of other convertible lenses such as protars, raptars and the turner-reich. Coverage, even for the requirements of contact printing, is rarely more than 40 degrees when using the single elements.

[Struan Gray]

Thanks for the reality check.

FWIW, my half Symmar is quite a bit better than the half Dagor (Berlin ser. III) and half Dialytes (APO-Ronar, APO-Lustrar) I've tried, not to mention the telescope objectives, spectacle blanks and crystal spheres :-). It may only be a polished turd, but it's a polished turd I can see myself using.