Which lenses can make decent convertibles?

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I am looking for an inexpensive solution for occasional 600 mm shots on 4x5. Although I have a Fujinon T600, the lens is 1Kg and will never leave my car's boot. I have now the bellows for 600 mm and I thought I might try to separate a G-Claron 305. The images produced by both the front and back elements look pretty good on the GG, focussed right into the corners and with no distortion. The front element produces a 450 mm and the back a 610 mm. It should be a light long lens and I think I will make some film tests for real performances.

What are your experiences in this field? Can any symmetrical 6 elements lens be used that way? What about an Apo-Symmar? Would the initial aperture of 22 be the best to use as for the whole lens?

Is there a formula that helps determining the new initial aperture of a converted lens? (f9/300=f16/600?)

-- Paul Schilliger (pschilliger@smile.ch), July 22, 2001


Further trials showed that the images produced by both elements have the same size with the Claron (approx. equiv. 500 mm). Only the bellows draw changes (450-600). So not long enough for me (I have a 450 mm). A 360 mm would make little difference, and there is nothing small beyond that can be converted as far as I am aware. ? And the sharpness of the separate elements that I tried is discutable and only film will reveal what is achieved at f22.

-- Paul Schilliger (pschilliger@smile.ch), July 22, 2001.


The 240mm Fujinon A (an apo Plasmat) is one of the slickest triple convertibe lenses you will find anywhere. 10"-15"-20".

I try this with every lens I buy and this one is by far the best one yet. Using it with an 8x10 camera, At 15" (front element)you need to stop down to F32 to sharpen the corners, at 20" (rear element) it's not a problem. Effective aperture is still the diameter of the iris (after optical distortion) divided into the lens-to-film distance. With this particular lens, I simply measure the bellows and calculate based on a 10" focal length,using the standard formula for bellows factor.

-- Bruce Wehman (brucewehman@home.com), July 22, 2001.

Thanks Bruce! I tested and it seems to make a very nice and tiny 440 mm. ...Sadly the C series does not yeld to that kind of manipulation, the 450 would have made a small 700 mm. Anyway, there is now a clue on what I shall look for! Thanks.

-- Paul Schilliger (pschilliger@smile.ch), July 22, 2001.


I am also very interested in this G-Claron converting approach. Please tell us more details when you have finished the testing. Thanks!

-- Yongfei (yongfeil@yahoo.com), July 23, 2001.

The films (120 in 6x12 format) are now on my light table and both the results from the Fujinon A 240 back element and G-Claron 305 front and back elements look pretty good besides the Fujinon C 450 reference shot, although not as sharp! (who thought they would beat it?). Resolution is plenty and there is no apparent problem at normal magnification. However, at very high enlargement ratio, color fringing appears, especially with the G-Claron. I would recommend to test the Claron on the full 4x5 frame for sharpness in the corners. Seems to me that it softens slightly towards the edges of the 6x12. I used f22 as working aperture. Testing of an Apo-Ronar 360 back element produced a sharp image too at f32, despite the strong aberration showed at full aperture. But I'm not sure it would provide for sharpness on the entire 4x5 format. The front element image is not good on my film.

The G-Claron 305 single elements produces an equivalent 530mm with 450 or 600mm of bellows draw (front versus back element) and the 240A back element frames the equivalent of a 430mm lens with 480mm of bellows (I runned out of film to test the front element which requires less bellows). The Ronar back element needed 720mm of bellows for an equivalent 650mm frame. Seems that the back elements of some lenses produced a slightly better image towards the edges, what is confirmed by Bruce's observations. Out of the still unknown results of the Claron in the corners of a 4x5, I think this is worth considering when on a tight budget or to limit the weight of the pack, especially since the long lens shots are rather scarce in regard of the cost and bulk of those lenses.

-- Paul Schilliger (pschilliger@smile.ch), July 24, 2001.

In general, any single cell will work best with the aperture stop in front of it, because that cuts out more light from the outer portions of the lens, and quenches so-called zonal aberrations more effectively. Also, if the single cells are asymmetric (and most of them are), you will get worse curvature of field one way round than the other.

It probably makes little difference with a lens with cells which are as symmetric as those from a G-claron, but if you're trying to make a triple convertible from a less symmetric lens it is well-worth trying the front element behind the iris, either by reversing the lensboard or by screwing it into the rear threads of the shutter.

-- Struan Gray (struan.gray@sljus.lu.se), July 26, 2001.

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