Helios-40 conversion?

I'm not Brian, but I'll answer anyway.

Do you need rangefinder coupling?

If you don't, it's very easy and reversible, just an adapter tube with M39 on both ends and the right length, and an LTM-M adapter. Replace the M39 on one end with M42 if you have a newer, black Helios-40-2.

If you do want rangefinder coupling, you need to design a new focusing helical with either a newly-cut sloped rangefinder cam follower and a rotating barrel (the original Helios-40 barrel does not rotate), or a set of stacked helicals to transfer the movement of the 85mm lens to a the 50mm-equivalent movement of the cam follower. A fair metalworking challenge and nothing you could have done on the cheap.

Also, the Helios-40 is a big honking heavy lens. I'm not sure I'd want to lug that around on a rangefinder body, I find it a challenge already on an SLR. Mounted to the Zenit-S the combination is very top-heavy, and the Zenit-S is pretty much a rangefinder's equivalent in size and weight. You really need the heft of a SLR behind it to balance the lens for a workable combination. I use it on a F2AS and a 5D, with which it's quite ergonomic, except that it's awkward to use with the Nikon's protruding viewfinder.
 
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Some follow-up thoughts-

1) I would buy a Canon 85/1.5 in LTM Instead. They are cheap compared the the Leica and Nikkor 85/1.5's, and about the same cost of a Helios-40 these days. The latter used to run ~$100. Not anymore.

2) Without taking one apart, it is hard to say. The only economical way to do it would be it the optics module of the Helios comes out of the focus mount as a complete unit AND would fit into the Canon 85/1.5 focus mount. IF it does, you would could find a Canon with bad glass and try the "Hack". I converted a Helios-103 into LTM using a close-focus helical for an Elmar. Dremel required to shave down the optical fixture of the Helios-103.

It's a long shot. The Canon is worth looking into as a substitute.
 
Thank you both for your replies.

Brian, I have been considering the Canon 85/1.5 but the only ones I have seen advertised are going for around $1000 so I wonder if I am better looking at the 85/1.8 at around the same money with a stirling reputation. Having said that I am very keen to try a sonnar type though. Any other cheap sonnars in the 75-90mm range you would recommend?
 
My experience with a Helios 40 is that it is a bit soft. If you don't need the extra stop, why not get a Jupiter 9 in ltm which already has the rangefinder coupling?
 
The Nikkor 8.5cm F2 is a Sonnar type lens, and goes in the $300 to $500 range in LTM.

Go for it. Very few air/gass surfaces, hard-coated, F2 is plenty fast.
 
I think the only difference is the weight of the barrel between chrome (and early black) versions and late black versions.
 
The helios40 and Canon 85 f1.5 behave quite differently. I have the Canon 85. It is much more tame than the helios. Personally I prefer the Canon. It has a dreamy look without the vomit inducing swirls. A Biotar 75 would be another option if you like the big swirly oof area. I tried to have one converted with RF coupling a couple years ago with no success before giving up. The optical unit is much smaller in diameter (the helios 40 is 66mm biota 55mm). BTW Zeiss made a handful of Biotar 75's in LTM. If you get very luck you can buy one for less than a Lux 75. I have one in M42 mount that I use on my Canon 5Dii. It has a look quite similar to the Helios 40 but much more tame in my view.

I've posted this before. '51 Slim Zeiss Biotar 75mm f1.5
3848072929_0df24b7ac8_z.jpg
 
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