Question about how rangefinder focusing actually works

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3rdrate

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So I understand the basic idea of how rangefinder focusing works. I get that by turning the focus ring, I'm adjusting the mirror angle casting the overlapping image until the two images overlap, and that focus is achieved by triangulation.

But what confuses me is how this system works in practice. I'm currently shooting a Canon 7 with an Industar 50mm LTM lens (and having a lot of fun!). Upon examining the lens mount on the camera, I see that there's sort of a metal nub that can be pushed into the camera. I also see that as I focus, a ring extends from the lens, obviously pushing this nub back and adjusting the angle of the mirror.

But this sort of system seems like it'd be prone to TONS of focusing errors with just a simple misalignment in manufacturing, wear over time, etc, etc. With just a slight twist on the focus ring, I'm literally covering multiple feet in distance. So what if my Russian lens made God knows and where is SLIGHTLY off in matching the correct ring extension that'd designate, say, a subject being 20 feet back vs. 30? What if the screw mount has worn a bit and overscrews after years of use, pushing the focus ring further back into the camera? Or what if the bit inside my Canon body is off by a fraction of a milimeter?

And yet my camera, which I bought on Ebay and haven't CLAed, takes perfectly great in focus pictures with the Industar lens. And while I'd like to chalk it up to craftmanship, something tells me I'm overlooking a piece of this that makes such errors unlikely. So what is it?!
 
A rangefinder is generally designed so that it can accurately focus the longest/fastest lens in the system it was designed for. For the Canon 7, I assume that would be the 100mm F2 or 135mm F3.5 lens. Shorter/slower lenses don't require as much accuracy in the rangefinder system so any misalignment or wear is more than compensated for.
 
Right - but in simpler terms, are RF lenses designed literally as: if the focus ring is signaling a 10-foot distance to subject, the rear lens ring depresses the camera body's nub (whatever that thing is called) a specified distance, in turn angling the mirror to a precise angle allowing the overlap in images?

It just seems like there's a LOT that can go wrong in that process!
 
3rdrate said:
So I understand the basic idea of how rangefinder focusing works. I get that by turning the focus ring, I'm adjusting the mirror angle casting the overlapping image until the two images overlap, and that focus is achieved by triangulation.

But what confuses me is how this system works in practice. I'm currently shooting a Canon 7 with an Industar 50mm LTM lens (and having a lot of fun!). Upon examining the lens mount on the camera, I see that there's sort of a metal nub that can be pushed into the camera. I also see that as I focus, a ring extends from the lens, obviously pushing this nub back and adjusting the angle of the mirror.

But this sort of system seems like it'd be prone to TONS of focusing errors with just a simple misalignment in manufacturing, wear over time, etc, etc. With just a slight twist on the focus ring, I'm literally covering multiple feet in distance. So what if my Russian lens made God knows and where is SLIGHTLY off in matching the correct ring extension that'd designate, say, a subject being 20 feet back vs. 30? What if the screw mount has worn a bit and overscrews after years of use, pushing the focus ring further back into the camera? Or what if the bit inside my Canon body is off by a fraction of a milimeter?

And yet my camera, which I bought on Ebay and haven't CLAed, takes perfectly great in focus pictures with the Industar lens. And while I'd like to chalk it up to craftmanship, something tells me I'm overlooking a piece of this that makes such errors unlikely. So what is it?!
Click to expand...


Yes, rangefinders can go out of calibration from wear, vibration, etc. Even expensive ones like Leica. Every RF camera I have ever owned needed its RF adjusted for accuracy. All were bought used and were 20-30 yrs old.
 
Basically yes. Its a simple mechanical linkage between the lens and the body. Not a lot can go wrong with it really. Rangefinder adjustment is required every now and then, but I've never heard of wear being a problem.
 
If you're seeking the ultimate in rangefinder accuracy, procure yourself a decent example of the pre-war Contax II or III. The built in meter on the III is almost redundant even if it's working, because it does not use modern film speed calibration: hence, I recommend the II. Not only do these have just about the longest base length rangefinder system ever manufactured (with the possible exception of the Kodak Ektra, a brilliant camera in its own right, but one that also has some flaws), their RF systems demonstrate a truly prodigious ability to remain in calibration, and are incredibly accurate. You can purchase 80 year old examples today, with rangefinders that have never been adjusted since manufacture, with calibration that is still absolutely perfect.

Stephen has some good pages of information about the topic of what makes for an effective, accurate rangefinder on his site. This page has an interesting comparison chart showing effective baselength of various cameras. The only thing I would add is that there is a noticeable omission from the chart--yes, the Contaxes are not included. Stephen does provide the relevant information for them in his page about the models, though, which you will find here. I'll quote the relevant excerpt from his page below:
"Instead of a moving mirror system that Leica used, and still uses, Zeiss used a more expensive system which Zeiss claimed was more accurate system. By rotating two glass wedges to achieve rangefinder focus, rather than a single mirror, Zeiss claimed higher rangefinder accuracy which would practically never go out of alignment. Rangefinder baselength was a gargantuan 90mm with a magnification of about .75, giving an effective baselength of an amazing 67.5 -- 8% longer than the Leica M3 and 15% more accurate than its contemporary Leica screw mount competitors."
​
Cheers,
Brett
 
The simple truth is that the best are amazing pieces of precision engineering, brilliantly designed and superbly executed. The worst are often surprisingly good too. Much like leaf shutters and mechanical watches. And, like leaf shutters and mechanical watches, they can and do go out of alignment. A lot of it seems to be down to luck, and how many beers Hans (or Igor or Katsuji or whoever) had with his lunch before assembling a particular camera.

With their insanely complex rangefinder linkages, Contaxes need to be even better built than Leicas. Shame about the shutters, the lens mount and the ergonomics.

Cheers,

R.
 
I think what may not be apparent is that back in the day, mechanical cameras of any quality cost a lot compared to what they cost now. Even consumer cameras like the Yashica GSN or a Konica C35 were expensive. They were finely made machines. You may also notice it's about 50/50 odds of getting a digital slr that manually focuses accurately these days.
 
The rangefinder principle is simple geometry, and was used by Naval gunfire for generations to get shells on target miles away. The larger rangefinders could accurately calculate range within about 100 yards at 10 miles away, the small camera ones can do to a mm or so. The depth of field is more than that, unless you are very close. And very close...the rangefinder stops working on cameras.

If it didn't work, they wouldn't have used rangefinders as long as they did.
 
Both my 35mm f2 ASPH and 50mm f1.4 ASPH have a brass helicoids (as far as I'm aware all Leica lenses use brass) but looking at Zeiss and Voigtlander lenses the helecoids are black in colour and I'm not sure what they're made from. I'm guessing Leica designed the camera/lens system choosing brass because of the self lubricating properties, so would using Zeiss or Voigtlander increase wear?
 
It seems to me that your question isn't about how RF focusing actually works but why it doesn't go off instantly and produce a lot of errors. Basically, it's as you said it: craftsmanship; quality design and materials; careful, accurate assembly and calibration; designs meant to tolerate a lot of use and abuse; occasional need for recalibration.

Unlike some others, I have found very very little need to have my Leica RF cameras' viewfinder/rangefinder serviced over the past forty some years. In fact, only one of my cameras—my current 1978 M4-2, bought in 2011—required service to clean, recollimate, and calibrate the rangefinder due to being neglected for likely about twenty-five years. A $100 service had it working perfectly again.

G

3rdrate said:
So I understand the basic idea of how rangefinder focusing works. I get that by turning the focus ring, I'm adjusting the mirror angle casting the overlapping image until the two images overlap, and that focus is achieved by triangulation.

But what confuses me is how this system works in practice. I'm currently shooting a Canon 7 with an Industar 50mm LTM lens (and having a lot of fun!). Upon examining the lens mount on the camera, I see that there's sort of a metal nub that can be pushed into the camera. I also see that as I focus, a ring extends from the lens, obviously pushing this nub back and adjusting the angle of the mirror.

But this sort of system seems like it'd be prone to TONS of focusing errors with just a simple misalignment in manufacturing, wear over time, etc, etc. With just a slight twist on the focus ring, I'm literally covering multiple feet in distance. So what if my Russian lens made God knows and where is SLIGHTLY off in matching the correct ring extension that'd designate, say, a subject being 20 feet back vs. 30? What if the screw mount has worn a bit and overscrews after years of use, pushing the focus ring further back into the camera? Or what if the bit inside my Canon body is off by a fraction of a millimeter?

And yet my camera, which I bought on eBay and haven't CLAed, takes perfectly great in focus pictures with the Industar lens. And while I'd like to chalk it up to craftsmanship, something tells me I'm overlooking a piece of this that makes such errors unlikely. So what is it?!
Click to expand...
 
What you describe actually can be a problem with digital rangefinders, due to the difference between the focus plane of a sensor as compared to the focus plane of gelatin films which can have varying thicknesses. The tolerances with digital are much more critical. In addition, the larger an enlargement, the easier it is to see focusing errors. Most people are not pixel-peeping their film images like is common with digital, so there might actually *be* slight mis-focus but it's just not apparent in a typical 8x10.
 
Bill Clark said:
...Autofocus cameras operate kind of like a bat that use echo to determine distance. ...
Click to expand...

That's only true for cameras that use the Polaroid Sonar Autofocus Sensor.

Most AF systems are based on contrast detection, whether directly as in CDAF systems or indirectly as in PDAF systems. A very few use a laser interferogram technique (some older Sonys as far as I am aware).

G
 
There can be issues when fitting the lens to the camera - I can't remember the exact term, but basically whether or not the lens finally sits the right distance from the film plane/face of its fitting. That aside, as said above, the problems you suggest are counteracted by good engineering. To give you another example, on my bike I've had two different cylinder heads and three lots of camshafts (with bucket and shim adjusters) and, provided you kept each shim with its valve, any combo gave the same valve clearances.
 
I think the designers of the M3 would yawn wearily at the original post here. Rather than surmise how much might go wrong, one should rather marvel at how wonderfully they executed such a demanding task and got it so right. The backward movement of the focusing cam is so precise in its displacement of the rangefinder roller arm that it was possible to focus a 50mm f1.4 lens wide open. Further, they could arrange for a very different 135mm lens focus helicoid to also displace the roller arm the same distance for focus between infinity and 3 meters or closer. That would require crazy precision engineering wouldn't it? Yes, it did.
 
Richard G said:
I think the designers of the M3 would yawn wearily at the original post here. Rather than surmise how much might go wrong, one should rather marvel at how wonderfully they executed such a demanding task and got it so right. The backward movement of the focusing cam is so precise in its displacement of the rangefinder roller arm that it was possible to focus a 50mm f1.4 lens wide open. Further, they could arrange for a very different 135mm lens focus helicoid to also displace the roller arm the same distance for focus between infinity and 3 meters or closer. That would require crazy precision engineering wouldn't it? Yes, it did.
Click to expand...
Dear Richard,

We need a "like" button.

Cheers,

R.
 
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