MIR-1B 2.8/ 37 Minimum focusing dist.=1m04!

Mike43 · Joined: 27 Feb 2016 Posts: 6 Location: France

Hello everyone,

A couple of months ago I bought a MIR-1B 2.8/ 37 (M42 mount) on eBay from Russia and, while it's a very good lens in every other way - fast, sharp, nice tone, pleasing out-of-focus blur, clean optics, its minimum focusing distance is much longer than the lens' normal 0.7m at 1m.04! I see black varnish dabbed around the retainer screw-holes, obvious signs of a servicing and re-lube of this 1986 lens - and why not? - but I suspect some mistake was made at the re-assembly stage and I'd really appreciate any suggestions or pointers from other members in order to correct the lens; at 0.7m, the lens's normal minimum focusing distance is already quite long, but 1.04m really reduces its usefulness. I should mention that I've tried the Mir on different M42 adapters for my NEX-type mount and the result is the same.
I don't have much experience in lens disassembly but I'm fairly used to precision work and probably have some of the right tools. It's still a relatively inexpensive lens, so I'm prepared to chance an arm and have a go at putting it right. I've seen Anne Valeria's post about the stages in disassembly of a faulty M39 version of the Mir-1 and all the helpful advice given, which is very useful but I can't seem to find anyone with a solution to a similar problem with inaccurate minimum focusing distance, ergo the new topic. I've already removed the grub screws from the focusing ring and taken it off without trouble, in spite of the varnish. Any ideas about where I should go from there would be most welcome.

1kgcoffee · Joined: 16 May 2014 Posts: 52

What camera are you using the lens on? On a crop sensor the minimum focusing distance will be about %50 more. On NEX this will be the case. Try using on a FF camera like the A7R and it will focus up to 0.7 metres. You can also use extension tubes to get closer focusing but you will lose infinity. If there is a problem this an easy lens to take apart with a spanner wrench.

Mike43 · Joined: 27 Feb 2016 Posts: 6 Location: France

Thanks for the reply, 1kgcoffee. My camera's a Sony A6000, the "crop factor" of the image is 1.5, but surely that is just the size of the image that the sensor is capable of rendering whereas a lens' characteristics - F stops and focal length never change, F8 is always F8 and 50mm is always 50mm, even though there's a good bit less of the fifty's picture than on FF. My other manual lenses focus down correctly to their minimum focusing distances, just as they did on my old OM1n. For instance, right now I have the Helios-44 on my camera; its min. focusing distance engraved on the barrel is 0.5m - holding it at 0.5m from the screen of my monitor and focusing the lens right down brings the print of this forum page into sharp focus. Therefore I still believe there was a mistake made in the assembly, or re-assembly of this otherwise good lens. Perhaps, like the Helios, there are 3 threads on the helicoid and, due to faulty registering, the barrel was engaged in the wrong thread. Even if I could take it apart - grub screws on the rear element collar are resisting so far - I would still have a 1-in-3 chance of getting it right, but with marking and elimination, it should be possible. Fun, if I ever get the collar off!

miran · Joined: 01 Aug 2012 Posts: 1367 Location: Slovenia

Mike43 · Joined: 27 Feb 2016 Posts: 6 Location: France

Miran : This statement is completely wrong. Sensor format has no effect on MFD.

Exactly. MFD is an optical characteristic of the lens, as is the rest of the focal length and as F-stops are, and it never changes, or never should, unless the lens has become faulty. An APS-C sensor covers a smaller area of the image projected than an FF one does but the lens' nature cannot ever change without adding further devices to it (tubes, adapter lenses). On such a sensor, the 50mm lens does not become a short telephoto, as one so often reads, there is no real magnification or foreshortened perspective effect, only the outer edges of the image are lost in relation to what would be projected onto an FF sensor; at least, that is my understanding but I may be wrong.

I also do not find the Mir-1B to be easy to disassemble, or perhaps my version has had one too many dismantlings; the tiny grub screws are made of a very soft metal and are under a layer of varnish. So far, I've only managed to remove those of the outer, knurled focusing ring and slide it off; however, those securing the rear element's collar are resisting all attempts and I'm afraid I may be damaging them. As I understand it, without removing this collar, there is no way I can slide the helicoid out of the barrel and attempt to push it back in on a different entry point to the thread, giving me back the correct MFD, hopefully.

Mike43 · Joined: 27 Feb 2016 Posts: 6 Location: France

Any useful hints about how to dismantle this lens gratefully accepted.

Nordentro · Expire: 2015-01-29

How is the infinity alignment? Does it go far above? Have you messeured the distance or have you read the metering scale?

visualopsins · Expire: 2021-06-22

Welcome Mike43!

I guess you have checked adapter thickness. Wide angle lens is more sensitive to thickness than telephoto. Difference between 37mm & 50mm Helios is not much, however. With only about 0.7m difference between lens marking & actual focus distance, a minor adapter thickness issue could cause that.

Does the lens focus far past infinity? That could indicate helical mis-threading as you suspect. Could also be too thin adapter.

More people will see your post during the weekend -- stay tuned for more responses from other knowledgeable members. Smile

EDIT: I'm not sure about Russia lens. Asahi lens marking indicate distance from film/sensor plane, not distance from front of lens...

Mike43 · Joined: 27 Feb 2016 Posts: 6 Location: France

Thank you for your replies Nordentro and Visualopsins. Yes I have tried infinity focusing and the lens' focusing ring keeps turning well beyond infinity so that even distant objects (+300m) go out of focus. This would seem to prove faulty assembly of this 1986 lens - or perhaps faulty re-assembly, because no complaints about the lubrication, it turns very smoothly. If only I could dismantle the thing....!

Nordentro · Expire: 2015-01-29

Sounds like a heli mis-threading then. I`ve never opened a Mir-1, but I guess it is a simple construction.

Mike43 · Joined: 27 Feb 2016 Posts: 6 Location: France

@Nordentro Yes, construction looks relatively simple but access is not so easy - rear element collar needs to come off to free helicoid, apparently, but it seems to be secured by has tiny, soft metal grub screws? Haven't made any progress beyond this point.