APO Ronar as convertible

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Has anybody tried for example 300mm APO Ronar only with the back or front elements? It should then be around 500mm is this correct? I just got thad idea because I remember me on a Leica lens 560mm f 5.6 wich also had only 2 lenses and it was a very sharp lens in 35mm. I will do a test next week!

-- Armin Seeholzer (armin.seeholzer@smile.ch), April 01, 2002


Armin, Please post your findings, It will be interesting to read what you discover. Steve

-- (agno3@eesc.com), April 01, 2002.


the Apo-Ronar has a strictly symmetrical design. Using either front or rear element will yield a focal length close to 600mm (not exactly, because of the spacing between the two elements). However, do not expect too much from this. The better the correction of a whole lens, the worse it performs "in half mode". The two elements are desgined to cancel the aberrations of each other.


-- Thilo Schmid (tschmid@2pix.de), April 02, 2002.

Ditto, Thilo. Armin, I had tried to separate a 360mm. The image was difficult to focus at full opening but the aberration was partly corrected at f22-32. The image center was pretty sharp, but rapidly softened toward the corners of a 4x5". The back element needs bellows much longer than the front element for approx. the same image. Color fringing was noticable.

-- Paul Schilliger (pschilliger@smile.ch), April 02, 2002.

In conection, does anybody (other than Mr. Grimes) know how to calculate the new aperture on single elements? Is there any sort of equasion so I can play with single elements without shipping to RI? Sorry for my ignorance--Last time I took an optics or physics class was 22 years ago!

-- jason (sanford@temple.edu), April 02, 2002.

By the way, the Fujinon A series make good convertibles. They are six elements. But careful when experimenting: it's easy to drop a small piece!

-- Paul Schilliger (pschilliger@smile.ch), April 02, 2002.

Hi Paul

Maybe my 300mm will be better. Could be its the newest APO design only 1 1/2 years old. And I use it only for B/W but for the testing I will also use a color slide film. But I will not find time for the test next week, but as soon as possible you here my answer on it!

-- Armin Seeholzer (armin.seeholzer@smile.ch), April 02, 2002.

In an answer to my own question, apparently < fstop= focal length / diameter > so in the case of Armin's lens, f64=300mm/4.69mm, so assuming (I guess one would have to measure distance from the lens to ground glass to be sure) that a single element has a focal length of 600mm, the marked aperture of f64 would now be f128 with the ‘new’ lens (f128=600mm/4.69mm). Does this appear correct?

-- jason (sanford@temple.edu), April 02, 2002.

Jason, this is probably not a big help, but when I made some trials I simply added two stops to the nominal value when the single element focal length was twice that of the original lens. The results were OK but perhaps not precise.

-- Paul Schilliger (pschilliger@smile.ch), April 03, 2002.

Hi Armin! Any test made yet?

-- Paul Schilliger (pschilliger@smile.ch), April 24, 2002.

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