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I just tried that, and I still can't see vignetting. I guess I just got lucky...

The step down ring arrived by post. Against my expectations, I can't see any vignetting in the viewfinder. Neither at full aperture or when stopped down to f/22. It isn't quite what I expected with as extreme step-down as 67mm -> 52mm is. Well, I guess I should take a test picture with and without the step down ring.

It sounds surprising that stopping down will make vignetting more prominent, usually vignetting is reduced when a lens is stopped down. Regardless, I bought a cheap step-down ring from eBay, I guess I can just test it with it.

Basically how badly using a step-down ring will cause vignetting if the lens is stopped down as well? Specifically, I have got a Mamiya-Sekor C 80mm f/1.9 lens, which has 67mm filter ring, and I would like to try IR photography in medium format. The problem is that my IR filter is 52mm, and as Heliopan filters aren't exactly cheap, I'd prefer to use a step down ring instead if I can get away with it. Now obviously it would vignet a lot if you used it at full aperture, but how about stopping it down to say f/5.6? If I have understood correctly, that would basically mean that only the centre third of the lens would be used, so would this reduce the vignetting to manageable levels?

Aaaand now it's working. I basically just set the frame spacing to max and it was just fine in the next roll. :)

Looks like it works. In the next film I tested it with, the frames overlap the same amount (about 2-3mm) throughout the film. So I guess it's time to adjust the frame spacing according to the calibration guide. :D However, that guide tells to test it by loading a backing paper to the camera, and advancing it four times when the back is open. However, the counter doesn't advance while the back is open, so wouldn't this change the amount of the film advanced? I tested this with both the back closed and open. F and I marks mean full and initial, respectively, and the leftmost marks are with back open, and rightmost back closed. So, with back closed adjusting frame spacing to max would be a pretty good amount, while it would be a bit much if you go with the measurement done back open. On the other hand, if you measure it back open, it should have been about right at the beginning, so I guess that can't be correct...

Actually I managed to get the frame counter to work. First I tried to bend the part of the hook that connects to the release mechanism, as I didn't find any other easily accessable way to fix it. Unfortunately it broke, as apparently it was some hardened metal and it couldn't take bending. But I managed to make a fix by gluing beer can aluminium to it, as the forces involved aren't that large. It even works, so I guess I'll test it with film...

Unfortunately Pentacon 6 seems to have different frame spacing mechanism than Kiev 60. If I have understood correctly, Pentacon 6 attempts to actually measure the amount of film that is advanced, while Kiev 60 just estimates the compensation based on the frame counter. Which is why it being stuck is a problem. But anyways, I think I might have found what's the issue. The spring being a bit loose doesn't seem to be it, but the mechanism which resets the counter when you open the back. It pulls the hook, which stops the frame counter, when you open the back. But it looks like it doesn't go all the way down even when you close the back, so that the hook doesn't properly connect to the gears in the frame counter wheel. The next question would be how to fix it...

No, it can't be that because the film spacing consistently increased towards the end of the film. If it was due to advance lever stop missing, it should have varied from one shot to the next.

I have read that page earlier, but it seems to be about adjusting the fixed part of the frame spacing, while counter affects compensating the amount of film that is already on the spool. As the latter is not working, the spacing increases towards the end of the film.

Looks like spring loaded hook which should stop the counter as long as the back is closed, is very loose. It also looks like the spring has been already bent by someone else...

I opened the camera, and at least the frame counter moves when the top panel is removed. But it still doesn't lock into the next frame, but just slides back... so there's something else wrong as well.

So I got a good deal on a Kiev 60, which has frame counter stuck, but which seems to otherwise work fine. However, it turned out that the frame counter also adjusts frame spacing, so currently besides not knowing how many frames are left in the roll, I end up getting only 10 instead of 12 frames on a roll, as without the compensation spacing increases towards end of the roll. Which is a bit annoying. So, does anybody know if it's difficult to fix it? Looks like the camera is a bit banged up around the counter, so perhaps with good luck it might be just stuck due to the top being bent? It does look like the counter tries to move when you wind the lever.

Looks like the shutter adjustments were out of whack. When I set three turns of tension to both, everything is now working.

Oh, and looks like I have problems with shutter capping at speeds of 1/125 and faster. It used to have some problems with capping earlier with 1/500 and 1/1000s, but it was only one edge in the picture getting darker. There's some visible dirt on the shutter curtains, I take it that they need cleaning? So, what would be the correct way to clean them, and presumably also lube them afterwards?