Another Contax G 45 conversion...

T

Thatspec

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Finally fed up with the waiting, and armed with motivation from some of the FM Alt guys;
http://www.fredmiranda.com/forum/topic/1159122/0
I've finally got my G45 opened up and am sizing it up for conversion to M with a quality helicoid, in this case from a Rokkor 55/1.7 (thanks Sebboh). So far it's looking, well, downright easy. Ultimately I'd like to add a RF coupling tab but that's where the less easy part starts. There won't be any way to add a completely circular and rotating RF coupling with this helicoid that could be custom shaped. A standard tab will have to suffice but meaning what? Obviously it's not a 51.6mm focal length, it's 46mm so it may or may not focus correctly across it's entire range relative to the RF. Can someone explain in relatively plain language how it is that a 90mm lens for instance (or any other focal length than 51.6mm) can be designed to focus correctly across it's entire range relative to the RF? Is it built into the optical formula?
Thanks in advance...
 
Thatspec said:
Finally fed up with the waiting, and armed with motivation from some of the FM Alt guys;
http://www.fredmiranda.com/forum/topic/1159122/0
I've finally got my G45 opened up and am sizing it up for conversion to M with a quality helicoid, in this case from a Rokkor 55/1.7 (thanks Sebboh). So far it's looking, well, downright easy. Ultimately I'd like to add a RF coupling tab but that's where the less easy part starts. There won't be any way to add a completely circular and rotating RF coupling with this helicoid that could be custom shaped. A standard tab will have to suffice but meaning what? Obviously it's not a 51.6mm focal length, it's 46mm so it may or may not focus correctly across it's entire range relative to the RF. Can someone explain in relatively plain language how it is that a 90mm lens for instance (or any other focal length than 51.6mm) can be designed to focus correctly across it's entire range relative to the RF? Is it built into the optical formula?
Thanks in advance...
Click to expand...
The focusing cam is... well... cammed. In other words, as the lens is wound in and out, the cam is shaped so that the cam follower moves by the same extent as for a 50mm.

Cheers,

R.
 
Thanks Roger. I'm not fully up to speed on all the terms but after taking a closer look at the 100/F2 Canon I think I see what you mean. There is (yet another) complex mechanical device (the cam) regulating the travel of the cam follower. I never looked closely at it before and assumed it moved 1:1 along with the optics. It looks like I'd have to be happy with the focus set for a particular distance (probably 3 meters), and see where the rest falls. Either that or find another donor helicoid that has fully circular engagement.
 
You can see how this is done in photo #8 of ebay item 321014504195 (a Contax 90/2.8, see how the lens RF cam is shaped). No connection with the seller.
 
If you can mount something to the rear of the lens, which will rotate with the lens as it moves back and forth then you should be in luck! I believe a sloped cam should suffice for your needs.
 
I'm open to other suggestions for a donor helicoid. The Rokkor was suggested in the FM thread but mainly for M mount on a Nex. It's a nice quality helicoid for any amount of money (and they're like 35 bucks), unfortunately nothing accessible internally that rotates when you turn the focus ring. That Jeiying 90 is very nicely done, I've purchased from him before, he always has interesting stuff. Wonder if he's done any of the other G's? http://www.ebay.com/itm/321014504195?ru=http%3A%2F%2Fwww.ebay.com%2Fsch%2Fi.html%3F_sacat%3D0%26_from%3DR40%26_nkw%3D321014504195%26_rdc%3D1

I'll definitely take a closer look at the original mount after seeing that. With the Rokkor everything can be reversed, his way looks like full commitment. I'll post pics when there's progress.

Thanks guys.
 
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