Hacking the 75mm f/1.8 Heliar

[Phil_F_NM]

Several days ago I realized that at the closest focus stop of the 75mm f/1.8 Heliar there is about .7mm of focusing cam depth available to travel for the RF cam follower to further interface with. Sorry, I'm meaning to say that the lens was possibly built with closer focusing in mind since there is ample space for extra travel of the cam.

So today I took apart the lens. Had it down to the two parts of the helicoid and saw where I need to cut an extra 6mm in order to allow the helicoid to turn a bit more and bring the lens to .7 or even .65m for close focus.

The thing that stopped me is my lack of a very narrow spanner wrench. Sitting about 20mm deep, between the outer helical threads and an inner sleeve which translates the motion of the helicoid to a stationary cam, is a brass lockring that holds the sleeve in. If the sleeve can be removed, the camera-side of the outer helical can be machined out by perhaps 6mm to allow the three stops to travel a bit farther and decrease the focusing distance.

Once the outer barrel was machined, the close-limit hard stop on the mount itself would have to be relieved in order for all that hard machining work to be worth anything (or the limit screw can just be removed.)

There are a few issues with this approach though.
*The cam is a floating unit that is moved inside the aforementioned sleeve with three nylon rollers. If the outer helical is machined to allow extra movement of the rollers, the roller channels would overlap and the cam would have to be held in place with a thin wavy washer to act as a spring in order for the nylon rollers to not "jump" when they travel past the manufacturer's original limits. I don't see this as a problem so much as a possible issue.

*The machining would have to be done rather precisely as the 3 rollers' movement doesn't so much control the rate of the movement of the cam but definitely affects the smoothness with which the cam moves. It looks as if just one roller would be enough to impart motion to the sleeve for cam movement so this would save a lot of work.

Sorry I didn't take photos to demonstrate it all. The lens is pretty simple to operate on in case someone wants to tackle it. If one were to have a focusing issue with the lens it is extremely easy to correct by adding or subtracting shims which reside just under the focusing cam which is held in by a lockring and easily removed with a normal spanner.

If I actually go ahead with this operation, I'll make sure to have a macro-capable camera nearby so I can document the process in photos.

Phil Forrest