I'm having a strange problem with the lens in question. This is specifically the somewhat rare Carl Zeiss Jena Prakticar 50/1.8, not the more common Pentacon Prakticar. I cleaned the oily aperture by full disassembly, cleaning individual parts. Very simple, the mechanism seems well designed and easily serviceable. I have the aperture unit back together and working seemingly fine in full range, but when I mount the back end of the lens, with the aperture control mechanism, the range somehow becomes limited. The blades move with each click of the aperture ring, but no matter what it doesn't operate in full range. I even have the original service manual for this lens and it suggests adjusting the aperture is by loosening and moving the linkage arm. I've tried that but it only shifts the limited range to another end. So at first it wasn't fully open at f/1.8, and when I adjust it to be fully open, then it doesn't fully close at f/16 (very clearly, it looks more like normal f/8). Otherwise it works fine, opens and closes snappily when moving the lever at the lens mount.
Like I said, by itself the aperture unit works in full range, no visible problems. The mechanism is constructed in such a way that reassembly is seemingly foolproof - using screws to hold the "sandwich" unit together and another set to mount it into the lens chassis. There's only one way you can do it, with no room for any adjustment so I don't see how anything could be wrong here - in theory any mistake should be *very* wrong and immediately obvious. The problem only shows up when I connect it to the bottom half of the lens, with aperture control - but I didn't touch anything there, and don't see any faults.
To make the matter more interesting, a while ago I had the exact same problem with another Jena Prakticar lens, the 50/1.4 - it has a very similar aperture mechanism. Back then I eventually gave up and sold it as defective. So clearly there's something general about this mechanism I'm missing and it's not just a thing specific to this example. I reassembled it two times and spent a lot of time looking it over and searching for any info online but came up with nothing. I'm not a professional but the number of apertures I successfully fixed on various old lenses goes well into double digits. But I'm really stumped with this one.
Marked in red, the only available adjustment according to the service manual.
The aperture without the control unit, fully closed and open.
The aperture control unit inside the lens mount.
The aperture control unit attached - set to f/1.8
Like I said, by itself the aperture unit works in full range, no visible problems. The mechanism is constructed in such a way that reassembly is seemingly foolproof - using screws to hold the "sandwich" unit together and another set to mount it into the lens chassis. There's only one way you can do it, with no room for any adjustment so I don't see how anything could be wrong here - in theory any mistake should be *very* wrong and immediately obvious. The problem only shows up when I connect it to the bottom half of the lens, with aperture control - but I didn't touch anything there, and don't see any faults.
To make the matter more interesting, a while ago I had the exact same problem with another Jena Prakticar lens, the 50/1.4 - it has a very similar aperture mechanism. Back then I eventually gave up and sold it as defective. So clearly there's something general about this mechanism I'm missing and it's not just a thing specific to this example. I reassembled it two times and spent a lot of time looking it over and searching for any info online but came up with nothing. I'm not a professional but the number of apertures I successfully fixed on various old lenses goes well into double digits. But I'm really stumped with this one.
Marked in red, the only available adjustment according to the service manual.
The aperture without the control unit, fully closed and open.
The aperture control unit inside the lens mount.
The aperture control unit attached - set to f/1.8