Mamiya Universal, 100mm, & Infinity Focus

For those familiar with Mamiya Rangefinder Universal or Super 23 cameras, I've run into a problem.

Some time ago, I purchased a 100mm f3.5 Mamiya rangefinder lens, and it's not focusing correctly at infinity. (Note, this has nothing to do with the rangefinder mechanism.) I've extended the lens (twist-pull- backtwist) to it's proper location, and then I set the focusing dial on the lens to infinity. However, checking with a ground glass attached to the back of the camera, an image at infinity is not in focus.

As we know, the 100mm lenses can be extended and retracted. Be retracting the lens just a bit, the image at infinity does indeed come into focus. (Of course, the image at infinity should be in focus with the lens fully extended.)

Can these lenses be adjusted to focus in the correct way?

If the image on the film plane focus matches the rangefinder, then it seems the lens collimation (or infinity stop) is not set. See if you can locate the service manual for the lens. Otherwise, if the infinity stop is beyond true infinity, I'd be happier than the other way around. Just be careful to 'focus' at infinity when desired.

I used to be really up tight about this on my lenses until I started using autofocus lenses. None of those stop at infinity, probably by design so the AF can zero in on infinity.

Be retracting the lens just a bit, the image at infinity does indeed come into focus

Click to expand...

Unfortunately, with that situation it will be impossible to take a sharp picture of a distant object.

For those of us not familiar it is described on page 7 of http://www.mamiyaleaf.com/assets/files/documentation/Univeral_and_Accessories.pdf
Now some camera techs set infinity at 100 feet as they assume you will not be shooting wide open and by f8 infinity will be in focus.
If the rangefinder is correct for other lens at infinity on a target at least 5000 feet away the suspect a lens element incorrectly installed or element spacing to be in error. A worn lens barrel at the extension is a strong possibility for spacing error.

Figured it out, at least the majority of it.

It's easy enough to remove the lens itself from the helical focus mechanism. Sure enough, there were two, thin shims between the rear of the lens and the helical focus mechanism. Removing both helped a lot, but it's still not perfect. It appears that stopping down the lens a bit brings infinity into reasonable sharpness.

Hopefully, this should be good enough.

What I'd really like to do is find the f2.8 version of this lens. (Kind of expensive, though.)

Thank you for your responses.

Neil Poulsen said:

Figured it out, at least the majority of it.

It's easy enough to remove the lens itself from the helical focus mechanism. Sure enough, there were two, thin shims between the rear of the lens and the helical focus mechanism. Removing both helped a lot, but it's still not perfect. It appears that stopping down the lens a bit brings infinity into reasonable sharpness.

Hopefully, this should be good enough.

What I'd really like to do is find the f2.8 version of this lens. (Kind of expensive, though.)

Thank you for your responses.

Click to expand...

At one time I've had two more or less complete Mamiya Press outfits. I had all sorts of problems with calibrating all lenses to critical focus. Basically every single lens has to be calibrated individually, except the normal 3,5 lenses. If the normal lens (100 or 90/3,5 mm) hasn't been tampered with, it should be used as the benchmark, as they cannot be individually calibrated (contrary to the 100 mm 2,8), to calibrate the rangefinder from. When that's spot on you can go on and calibrate the other lenses, both the long and the shorts. It was somewhat rewarding, when I managed after too much toil and trouble, but I wouldn't want to try again (mostly due to old age, perhaps). But good luck! - common sense and a bit of hard work should do the trick.