a Frankenstein Topcor 90mm f5,6 ?

[removedacct2]

this duplicates a post on largeformatphotography.

I bought a Horseman 985, my first technical camera, thinking of using it mostly 6x9 and a little bit of 4x5. it came with the 4x5 groundglass extension and with a Topcor Super 90mm f5,6.
I have been so much distracted with this being my first technical camera that I didn't notice until today that the shutter is wrong:

silly....
the manual has the specs for the lenses:
https://www.cameramanuals.org/prof_pdf/horseman_985.pdf

this looks like the shutter of the Pro 105mm f3.5

I was wondering if the lens is in fact a Pro 105mm f3.5 with the front of a Super 90mm f5,6 but I see no notches for a spanner in order to unscrew the front and check is there's more glass ( the Pro 105mm, faster has a bigger diameter front element)

the Pro 105mm f3.5 image circle @f22 is 120mm, the 90mm @f22 is 150mm so just enough for 4x5 with slight vignetting.

I took two shots with the 4x5 extender, indoors, at f8, and there's vignetting, no idea if it matches the one a Super 90mm @f8 would produce, but the issue is the large blurry tunnel effect.

on largeformatphotography forum it has been suggested the front element could be indeed of a Super 90mm, but since the shutter isn't the one for the lens, maybe the back element too.

 

[Dan Fromm]

this duplicates a post on largeformatphotography.


I took two shots with the 4x5 extender, indoors, at f8, and there's vignetting, no idea if it matches the one a Super 90mm @f8 would produce, but the issue is the large blurry tunnel effect..

The shutter looks like a perfectly fine shutter for a 6x9 Horseman camera on a perfectly fine board for a Horseman camera. It is the wrong shutter for an f/5.6 lens. Chances are the f/5.6er's cells were swapped into the wrong shutter, or perhaps just the front cell was put into the wrong shutter by a sloppy owner, repairman or seller.

The 4x5 extender shouldn't vignette a 90 mm lens.

Check the lens in shutter to see if mechanical vignetting is a problem lens itself or with the extender. Count reflections to see whether the rear cell is the correct formula for the 90/5.6. You should see 4 strong reflections and 1 weak (often hard to see) if the rear cell is the right formula. If it is right for the 105/3.5 you should see 4 strong and no weak (if the sketch in the manual you linked to is correct, I doubt it because reversed tessar lenses are very uncommon) and 2 strong and 1 weak (possibly hard to see) if the diagram is reversed. If you don't know how to check for mechanical vignetting or count reflections, please ask.

 

[removedacct2]

Check the lens in shutter to see if mechanical vignetting is a problem lens itself or with the extender. Count reflections to see whether the rear cell is the correct formula for the 90/5.6. You should see 4 strong reflections and 1 weak (often hard to see) if the rear cell is the right formula. If it is right for the 105/3.5 you should see 4 strong and no weak

- mechanical vignetting, it's easy: I lock the lens wide open and look through it. I can see the corners of the 6x9 GG and also the 4x5 extender. So no mechanical vignetting.
- reflections, with a tilted small torch carefully used I spot 4 , not sure there's a weak one .... but unsure I do it well

i tested it before on 6x9, all good and sharp across all the frame corner to corner.

 

[ic-racer]

Your lens should look like this, as I think you and Dan have figured out.