Adjusting shoe-mount rangefinder - infinity target?

jay moussy · Jul 28, 2022

I got a shoe-mount rangefinder, and it seems it is not quite right with, say, a telephone pole three houses away.

What would be the appropriate distance of target to be considered at "infinity", for this task?

madNbad · Jul 28, 2022

Something much farther away, like the moon. The telephone pole may seem far enough but not quite. I always thought my rangefinders were off until I focused on the moon one night.

Sirius Glass · Jul 28, 2022

jay moussy said:
I got a shoe-mount rangefinder, and it seems it is not quite right with, say, a telephone pole three houses away.

What would be the appropriate distance of target to be considered at "infinity", for this task?

The Graflex and Graphic repairman who sold me the best of his collection recommended 1/4 mile [~0.5 km] as a practical infinity.

BrianShaw · Jul 28, 2022

For photography, infinity should be in “air quotes”… if not actual punctuation quotation marks. Some knowledge folks have claimed that 30 to 50 feet is acceptable.

madNbad · Jul 28, 2022

Somewhere between fifty feet and the moon. So your telephone pole three houses away should be good enough.

jay moussy · Jul 28, 2022

BrianShaw said:
For photography, infinity should be in “air quotes”… if not actual punctuation quotation marks. Some knowledge folks have claimed that 30 to 50 feet is acceptable.

Well, what am I going to do with the "100 ft" marking on my rangefinder

Besides, this is my second one, and it seems they are not very linear, meaning accuracy will vary across the distance scale. I may be doing it all wrong, has happened before!

BrianShaw · Jul 28, 2022

… but what rangefinder are you using? Some of the older auxiliary rangefinders seem to be rather low resolution. I don’t use them to determine infinity but more for a rough notion of closer( 50, 25, and 7 feet) distances. That’s where DOF can make more of a difference. And for the camera that I use it with most often, the range focus settings are. In fact, I don’t recall ever checking mine at infinity… I’ll have to do that just to see what it says.

BrianShaw · Jul 28, 2022

Jay… we were typing at the same time. I’ve had the same experiences! I bought 3 and was given one in parts before I got one I could really use. Most suffered other problems that precluded accuracy checking.

albada · Jul 28, 2022

For an accurate-enough infinity, the distance will be a factor of the distance between the two windows of the RF. If the window-separation is 50mm, for example, and you multiply it by 2000 (based on my intuition), you get 100 meters. Lemme calculate that in feet ... it's 328 feet. Or use the moon if it's not cloudy.

Hunter_Compton · Jul 28, 2022

Keep in mind that "Infinity" means different things for different lenses. For a 35mm lens on a 35mm camera, infinity might be 50 feet. Whereas for a 135mm lens on the same camera, infinity could be 500 feet.

This is more important when you have an interchangeable lens camera or a fixed lens rangefinder camera, as you can correlate infinity to the lens or lenses that will be used.

For an external rangefinder, it's probably best to align for the longest possible lens in use, or longest possible distance based on your geography. I have a water tower visible that's about 1/4 mile from my house that I normally use during the daytime, the moon is good at night as others have said. Radio/cell towers can also be good targets.

BrianShaw · Jul 28, 2022

I have a vague recollection that some say infinity is 200x (EDIT: not 20x as originally written) focal length.

Hunter_Compton · Jul 28, 2022

BrianShaw said:
I have a vague recollection that some say infinity is 20x focal length.

Equating millimeters to feet/yards/meters? Because 20x the actual focal length would be very short. On a 50mm lens, that's only 1 meter.

reddesert · Jul 29, 2022

There are ways to quantify this. The accuracy of a rangefinder is set by: the baseline of the rangefinder, its magnification, and the minimum angle your eye can resolve. For example, suppose you have a rangefinder with baseline 50mm and finder magnification 0.7x, and your eye can resolve an angle difference of 1 arcminute (which is 1/3438 of a radian). Think of a triangle where the short side is the effective baseline of the RF (baseline*mag), the two long sides are the distance from the object to the RF windows, and the tiny opening angle is 1 arcminute. Thus, the maximum useful distance of the rangefinder is D:

D = baseline * finder mag / minimum_angle = 50mm * 0.7 / (1/3438) = 120 meters.

So for these numbers, which are about right for a typical rangefinder camera or small aux rangefinder, 120 meters or about 400 feet is the farthest distance you could hope to resolve; any further and it might as well be infinity. There is probably a slop of a factor 2 in here depending on your vision and other details (compact RFs have smaller baselines and finder mag, etc), but essentially, a city block is about enough; 100 feet is too close to be infinity.

It's also possible to figure out how accurate the RF needs to be to focus a lens, using the required extension and depth of focus; this depends strongly on focal length, it goes as focal length squared. Which is why most RF systems max out around the equivalent of 135mm on 35mm format.

Helge · Jul 29, 2022

Don’t worry about infinity. Just set the lens to infinity.

What you should worry about is the near distances, especially the precision of the very close distances.
After ten meters DoF will take care of itself as long as you are not completely inept at judging.

Try moving your eye to see how much that changes the coincidence.

BradS · Jul 29, 2022

The moon.

gone · Jul 29, 2022

100' is fine, 50' probably would do it. There is really no difference in focus between 100' and the moon, it's the same to the camera.

More important is the close up distance, although if infinity is good that all should be OK. I usually measure some distances and calibrate that to my RF distance.

Huss · Jul 29, 2022

Da moon

BrianShaw · Jul 29, 2022

Hunter_Compton said:
Equating millimeters to feet/yards/meters? Because 20x the actual focal length would be very short. On a 50mm lens, that's only 1 meter.

Ahh, yes… my recollection was close and my typing was typographical. 200 (two hundred) times focal length. So said the somewhat controversial Steve Simmons.

I’ll correct my posting, above. Thanks very much for double checking me!

grain elevator · Jul 29, 2022

200 x f would still be too little. 10m for a 50mm lens? 2000 x f should do it.
I've used antennas on a building about 2-300m away for lenses up to 135mm. It's simple, if you can't see a difference in focus from infinity focus, be it with a rangefinder or an SLR, it's far enough, because you'll use that very system including your eye to do the actual focusing, so any precision beyond what you can see is useless.

MattKing · Jul 29, 2022

grain elevator said:
It's simple, if you can't see a difference in focus from infinity focus, be it with a rangefinder or an SLR, it's far enough, because you'll use that very system including your eye to do the actual focusing, so any precision beyond what you can see is useless.

Be careful with this statement - particularly the part I've marked bold.
When I said essentially the same thing about darkroom grain focusers and a sheet of paper on the easel, it led to a very, very, very long thread!

Adjusting shoe-mount rangefinder - infinity target?

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