a fake Topcor 90mm f5.6 (Horseman)?

[landstrykere]

"funny" lens...
I had not time today for playing a bit with this lens, but I took anyway two shots in 4x5 with the extender, back home.
Retropan-320 @640 iso
1st picture full open f5.6, 2nd picture f16
developed in HC-110 1+62, 25mn, 30s agitation then 10s every 2 mn.
side to side f5.6 - f16



100% of center:



100% of a corner:




for sure no vignetting and in fact somewhat usable from f16. Have to play more with it f16 to f32 outdoors, landscape, portrait, etc.

In the meanwhile I bought a Topcor Super 105mm f4.5

[reddesert]

Length and weight were the first things I though of, length is ~39,60mm and weight ~225g, the 90mm lens is 220g and 40,5mm long as per manual and the 105 f3.5 240g, length of 90mm and 38mm.

oh well, now I focus on this possibility. Because looking at pictures of 105mm f3.5 online besides the diagram in the manual, I see the rear element is very different than the one I have, which looks more like the 90mm.

mine:

Attachment 214253

a real Topcor 90mm (checked many onine listings):

Attachment 214254

a real Topcor Pro 105mm f3,5:

Attachment 214255

the rear element is clearly very different. much smaller on the 105f3,5.

so what I have may be a 90mm which shutter was broken and replaced with the shutter of a 105f3.5, but doing so, spacing may differ...

whatever it can be, so it is, nothing I can do unless I can grab a broken 90mm with working shutter and swap elements... The lens anyway performs very well on 6x9...

Okay, these pictures are good and clear some things up. The lens you have seems to be the proper front and back of the 90mm. And the 105/3.5 has a larger front cell than rear cell, suggesting it is a regular Tessar. The Horseman manual just drew the 105mm lens diagram backwards. That means the 105mm rear cell should be a cemented doublet, so you'd only see two reflections. I believe you don't have that.

Something you can try, since Horseman was kind enough to specify the 40.5mm total length of the 90mm lens, and yours measures 39.6mm, is just unscrew the front or rear cell to move it out by 0.9mm and restore the original spacing. Because Copal #0 shutter thread have a pitch of 0.5mm, you need to unscrew by ~1.8 turns. See if that improves the off-axis image quality.

There is also a recent thread in which someone mentioned using critical magnification on the ground glass to adjust the spacing of an Apo-Ronar.

If you find a spacing that works, then you can make a small shim to put under the element to replace the one that somebody must have lost.

FWIW, although it looks pretty certain that you have both 90mm lens cells, you could also measure the focal lengths of the single cells and try making images with them as Randy suggested. The 90mm is a plasmat and a single cell of a plasmat is usually 1.4-1.8x the focal length of the whole lens. The 105mm is a tessar, and the rear cell of a tessar has all the focusing power - the front cell won't make an image.