Schneider-Kreuznach Retina-Xenon 50mm f/1.9 Lens Calibration

I recently did a bit of a disassembly, cleaning, and lubrication of this lens that came with my Retina IIIS and now need to calibrate the focus to the rangefinder on the IIIS. I'll be using a split prism focusing screen scavenged from a non functioning Minolta X300. From what I understand, the basic process is:

1. Put the camera on a tripod facing something far away with the back open and set to bulb.
2. Tape or hold the focusing screen on the film plane.
3. Hold the shutter open (I might use a shutter cable that has a lock)
4. With the lens set to infinity, use a loupe to check precise focus at the screen and adjust how the helical sits in the camera body as needed.

My general question is, am I missing something or are there things to especially watch out for?

Most sources seem to say something along the lines of focus on "something as far away as possible." In the real world does this mean find a place that I can focus on something a mile away or would choosing a light pole 150 meters out my window be plenty?

Also, there seems to be a slight rattle I can't get rid of between the two main parts of the lens body. The link between them is the one with the large teflon washer and the two brass curved retaining pieces. Even with adjusting the brass pieces pushed as far out from the center point there is still the slightest movement. It rotates smoothly and I'm guessing it won't effect functionality but just curious if I should try adjusting it again.

Thanks for any info!

I read somewhere (can't remember where) that 'infinity' to a given lens is 'greater than 1000x the focal length of the lens'. For a 50mm lens, that works out to 50m / ~160 feet. So that 150m should give plenty of safety margin.

It might be more convenient to set up an pseudo autocollimator using a digital camera and work from the comfort of your home.

1. open the shutter on B
2. take a 35mm tall piece of glass, put a sharpie mark on it and hold it on the film plane (mark facing lens)
3. setup a digital camera with a (preferably) long lens, make sure it is focused at infinity
4. place it in front of the Xenar lens and observe the sharpie mark using the zoom on the screen. It'll come into sharp focus when the Xenar is at infinity.

Your suggested method also works. It's best if you can sight a tall building against the sky. Sometimes there is a small parallax error using a split prism but it's minuscule.

nosmok said:

I read somewhere (can't remember where) that 'infinity' to a given lens is 'greater than 1000x the focal length of the lens'. For a 50mm lens, that works out to 50m / ~160 feet. So that 150m should give plenty of safety margin.

Click to expand...

Oh good that’s great. I had looked up the hyperfocal distance and it wasn’t super far so I was curious why people seemed to think miles and miles were better

OAPOli said:

It might be more convenient to set up an pseudo autocollimator using a digital camera and work from the comfort of your home.

1. open the shutter on B
2. take a 35mm tall piece of glass, put a sharpie mark on it and hold it on the film plane (mark facing lens)
3. setup a digital camera with a (preferably) long lens, make sure it is focused at infinity
4. place it in front of the Xenar lens and observe the sharpie mark using the zoom on the screen. It'll come into sharp focus when the Xenar is at infinity.

Your suggested method also works. It's best if you can sight a tall building against the sky. Sometimes there is a small parallax error using a split prism but it's minuscule.

Click to expand...

I don’t have a digital camera (other than an old point and shoot) or I would try that method for sure! It’ll have to keep it mind for the future. I might be totally off base with the science behind it, but would it work with an SLR with a telephoto lens?

Interesting to hear about the parallax error. I’ve found myself thinking like a perfectionist in this project but then I have to remind myself that my own error is likely to mess up the focus more than any mechanical error will, whether it’s calibrating or actually taking a picture.

nosmok said:

I read somewhere (can't remember where) that 'infinity' to a given lens is 'greater than 1000x the focal length of the lens'. For a 50mm lens, that works out to 50m / ~160 feet. So that 150m should give plenty of safety margin.

Click to expand...

sounds reasonable!

AKodakZen said:

I don’t have a digital camera (other than an old point and shoot) or I would try that method for sure! It’ll have to keep it mind for the future. I might be totally off base with the science behind it, but would it work with an SLR with a telephoto lens?

Interesting to hear about the parallax error. I’ve found myself thinking like a perfectionist in this project but then I have to remind myself that my own error is likely to mess up the focus more than any mechanical error will, whether it’s calibrating or actually taking a picture.

Click to expand...

Yes it would work with an SLR. You make a fine vertical mark on the glass (or scratch a piece of clear film and sandwich it between rails and glass) and check focus with the split prism of the SLR. I find that the zoom on a screen is more precise vs. the small VF.

You can check for parallax error by moving your eye laterally. The vertical line will split a little bit.

OAPOli said:

Yes it would work with an SLR. You make a fine vertical mark on the glass (or scratch a piece of clear film and sandwich it between rails and glass) and check focus with the split prism of the SLR. I find that the zoom on a screen is more precise vs. the small VF.

You can check for parallax error by moving your eye laterally. The vertical line will split a little bit.

Click to expand...

That seems like it worked. Thanks for the info. Really cool process process. I’m going to compare results to the focus screen method and see how they compare out of curiosity.