Busch Rapid Symmetrical f8 not covering 4x5?

I bought a Busch barrel lens. Only markings are on the flange "Busch Rapid Symmetrical f8". It has an iris diaphragm scaled on the barrel f8 to f45. No mount flange. The lens isn't on the listing at http://www.earlyphotography.co.uk/site/entry_L26.html , where similar yet not identical lenses are mentioned.
It may be the one mentioned on this old thread:
https://www.photrio.com/forum/threads/busch-rapid-symmetrical-f-8.34486/

anyway here is the one I have:




front element screws off easily, not the back:






it is convenient, I can mount it on front of a Copal or Seikosha #0 shutter. Here fixed just with gaffer:



bu I have also hacked a board, so I tested it using a lens cap as shutter. I had an old Pentax cap that fits perfectly:




I found older auction pages picturing this lens on camera. For instance:



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I hadn't time to go around play with it but took some shots indoors.
I am shocked it doesn't fill 4x5. I use a Horseman technical with the 4x5 enlarger back.

a comparison with the Topcor-150, and the Schneider-Kreuznach Tele-Arton 270mm, all closed down (f45 for the Topcor and Busch, f32 for the Kreuznach). Top to bottom Topcor, Arton, Busch:





with the Busch (used on lensboard not on front of a shutter so to be sure no light is cut by the thickness of the shutter), quite a lot of wasted space.



I don't get it, I was expecting this lens to fill 4x5. Unless Busch made a variant for 6x9.

and forgot to mention, I checked for vignetting by looking at the inside of the camera from the front of the lens wide open, and I can see the 4x5 ground glass corners... so I don't get why it doesn't fill the frame.

First, you have to measure the focal length of the lens in order to get an idea of what it should cover. You can measure the apparent aperture size and multiply by the f-number, or measure the distance from center of lens to film when focused at infinity. Neither is exact but both will be close enough. Just by rough comparison with the other lenses's field of view it suggests 210-220mm or so (if you didn't move the camera).

Next, the dark vignetting in the shot above is rectangular. Lenses are round (circularly symmetric). When lenses don't "cover" a format, what we mean is that either the lens image quality falls off or the illumination falls off, and these are circularly symmetric patterns. Meaning blurry or dark corners. The lens can't make a rectangle. That has to be mechanical vignetting by the camera.

reddesert said:

First, you have to measure the focal length

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. When you look at the pictures, i did put a piece of paper with the name of the lens and its focal, on the upper shelf in the middle, for each shot, so I could tell what picture with what lens after I developed the negatives ... You can see that the 3rd shot has "Busch 210" written on that piece of paper in the shelf ...

[qiote]the dark vignetting in the shot above is rectangular. Lenses are round (circularly symmetric). When lenses don't "cover" a format, what we mean is that either the lens image quality falls off or the illumination falls off, and these are circularly symmetric patterns. Meaning blurry or dark corners. The lens can't make a rectangle. That has to be mechanical vignetting by the camera.[/QUOTE]

what i don"t get is why there's mechanical vignetting only with the Busch ... Any idea?

antonio_b said:

what i don"t get is why there's mechanical vignetting only with the Busch ... Any idea?

Click to expand...

The best way to find out would be to look at things from behind the camera. If your ground glass has cut-off corners, you can observe the aperture through them to look for vignetting. If not, maybe you can remove the ground glass, or the whole back, to see what's happening.