Bottomloading cameras -- calibration?

RLangham · Oct 6, 2020

So for bottom-loading cameras that don't have a hatch in the back like Leica M's or a a little inspection plug like some Barnack Leicas, are there any good solutions for viewing through the lens for rangefinder calibration? I'm just curious about how this is regularly done, since I don't think my Zorki really needs it.

I know how to take the body off my Zorki but then obviously there's no film guides to put ground glass or matte tape across. If I ever have another bottom-loader, how would I do it?

Kino · Oct 6, 2020

I think the way to do that is kind of complicated and you'll need some tools.

I would think you need to determine critical back focus distance on your lens, from the flange top, and then confirm the focus via a depth gauge caliper with film loaded in the body.

I saw an article on this once and the technician had to remove the spring from the depth gauge micrometer to keep it from influencing the measurement, as it would push against the pressure plate and mess up the reading...

Donald Qualls · Oct 6, 2020

You can check the lens on a known-good camera body, and then (with a springless depth micrometer, as noted above) check the flange to film distance with the lens off. At the factory service centers, they probably had a fixture for verifying lens focus.

ic-racer · Oct 6, 2020

Use film and bracket the focus. Keep notes.

E. von Hoegh · Oct 6, 2020

RLangham said:
So for bottom-loading cameras that don't have a hatch in the back like Leica M's or a a little inspection plug like some Barnack Leicas, are there any good solutions for viewing through the lens for rangefinder calibration? I'm just curious about how this is regularly done, since I don't think my Zorki really needs it.

I know how to take the body off my Zorki but then obviously there's no film guides to put ground glass or matte tape across. If I ever have another bottom-loader, how would I do it?

If you remove the body shell from your Zorki, (and most any other bottom loader) you lose the lens mount. Film rails are still there, but the pressure plate is gone.
So. You must first verify the 28.8mm flange distance, this is easy if you have a depth mike and are competent in it's use.
There seems to be confusion between "micrometer" and "dial indicator". This often results from yootoob videos. A "mike" - short for micrometer - has no spring and relies on "feel".
So, now you have the flange precisely 28.8mm from the film plane. You measured the center and each corner. You install a lens that is known to be properly calibrated and zero the rangefinder at "infinity", but you still don't know about the shorter distances. So you have to use film, set up some test targets at say minimum, 2m, 5m, etc. and adjust the slope accordingly, slope adjustment varies on different cameras - on a Zorki-S it's the skid follower, on most others it's the roller. The test targets should be in the center of the frame and consist of a zero (focussed distance) and at least one in front, one behind - more targets on either side of zero are better, lens of course at maximum aperture.

beemermark · Oct 7, 2020

The lens is calibrated to the rangefinder , done simply by putting the camera on tripod and aiming at a street light 100 yds or more away and then adjusting the rangefinder. The rangefinder mechanism is calibrated/designed to the film plane and I think it would be incredibly hard to adjust if even possible.

summicron1 · Oct 7, 2020

beemermark said:
The lens is calibrated to the rangefinder , done simply by putting the camera on tripod and aiming at a street light 100 yds or more away and then adjusting the rangefinder. The rangefinder mechanism is calibrated/designed to the film plane and I think it would be incredibly hard to adjust if even possible.

I was about to say -- the folks who make the camera do the hard work and sync/calibrate the rangefinder to the film plane for you -- all you have to do is make sure it is focusing at infinity properly and the rest should fall into place. Anything else would mean the lenses would each need to be individually calibrated to every camera you put them on.

So don't go taking the cameras apart. If the lens and rangefinder both show infinity at infinity, you should be good. If they don't, you adjust the rangefinder with the lens on and set at infinity until they do.

reddesert · Oct 7, 2020

Possibly the way this is done by a technician is to use an autocollimator to focus the lens at infinity with the shutter open and film in the camera. A collimated beam of light from the autocollimator is focused by the lens onto the film, and reflected back out the lens. If the lens is at infinity, the exiting bundle of rays is also a parallel bundle of rays, and is focused by the autocollimator so you can verify it. Of course, the autocollimator itself has to be focused correctly.

Once the lens is focused at infinity, the rangefinder can be adjusted to focus at infinity. (Yes, many RF bodies do have an adjustment, sometimes a screw that can be reached through an access panel or screw hole, sometimes it's under the top cover.)

Here's an example of a home-built autocollimator (the author is a Photrio member): https://fixfilmcamera.com/Collimator/collimator.htm
Here's a variant of a Rick Oleson method to use an SLR as the autocollimator (of course, you have to know that the SLR is properly focused): http://feuerbacher.net/photo/repair/InfinityFocus/InfinityFocus.html

E. von Hoegh · Oct 7, 2020

Never mind... (sigh)

Kino · Oct 7, 2020

summicron1 · Oct 9, 2020

E. von Hoegh said:
Never mind... (sigh)

well, i think the bottom line here is don't sweat it.

E. von Hoegh · Oct 9, 2020

Don't sweat - true.
However, the OP did ask a good question. The autocollimator is a good solution, and I've used it, however it still relies on the lens-camera relationship. We don't have the rescources the manufacturers do; bottomloaders are a PITA this way. If you can put a focussing screen on the film rails, it's easy - some rfs let you do this. The method I described relies on a properly calibrated lens, as does the autocollimator. Think about "infinity" focus - it's unattainable, light from an infinitely distant object takes an infinite time to arrive. And then, there's a quibble regarding the definition of "infinite".
I prefer the moon to set infinity, as a rough & ready check the castle on Whiteface is 8 miles as the crow flies, but Luna is my standard. We're talking rangefinder baselines of say four inches maximum, and small reproduction ratios on the film. A target one-quarter million miles distant will do very well.

Kino · Oct 9, 2020

Sorry, I am the one who carelessly mixed up the terms for depth gauge and micrometer.

I was trying to remember an article from a book by Isaak S. Maizenberg on repairing Russian cameras in which he uses a modified dial indicator with the spring removed to fashion a depth gauge for measuring flange to film plane depth.

Bottomloading cameras -- calibration?

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