rangefinder accuracy

[alice]

Hi all,

just an issue about checking rangefinder accuracy.

For infinity basically its checking if the infinity is correct at something that is a mile away.

For close up I have read somewhere about putting a piece of black tape on a mirror, focus it so it is 1m away, and then shift over a little and refocus on the camera reflection in the mirror and it should read 2m....?

are the above ways correct in terms of checking the rangefinder accuracy?

the reason why i asked is that i have recently run into problems focusing with my MP & 50/2 cron at close distance at F2, i used the above methods to check and it seems like everything is fine and i am now beginning to question if it is my eye that is causing the problem when it comes to focusing....... i mean it shouldn't happen at F2.....


advise? thanks in advanced.

 

[richard_l]

FWIW focussing is most critical at wide apertures, such as f/2, and close distances, because of the shallow depth of field. The best way to check near-focus rangefinder accuracy is probably to put something like a piece of ground glass in the film plane and observe directly through the back of the camera whether the image is in focus when the rangfinder indicates that it is. Personally I don't like monkeying around inside my cameras that much and would rather leave it to a Leica expert.

 

[pvdhaar]

One way to check is to use a ruler with an object placed at exactly 1m. After focussing, the lens barrel marking should line up at 1m as well. Note that you have to measure this 1 meter from the film plane.

If your camera is correct, then you need to look at your technique; do you keep your eye centered behind the viewfinder? If something has crept into your way of shooting that causes you to view at an angle, then that may be a source of focussing error.

 

[waileong]

How?

How?

How would you measure something exactly from the film plane? That's the challenge. Obviously, you don't want to touch the shutter curtain, which would mean that you would want to have the camera on a tripod, depress the button fixed on B, have a meter rule touch the film back and place an object at the end of the meter rule.

Too many things could go wrong, eg. you have to ensure that your meter rule is exactly perpendicular to the film plane (else, it won't be exactly 1 m), and you better make sure the shutter doesn't close while you stick the meter rule through the throat of the camera. I wouldn't want to take that kind of risk.

Wai Leong

One way to check is to use a ruler with an object placed at exactly 1m. After focussing, the lens barrel marking should line up at 1m as well. Note that you have to measure this 1 meter from the film plane.

If your camera is correct, then you need to look at your technique; do you keep your eye centered behind the viewfinder? If something has crept into your way of shooting that causes you to view at an angle, then that may be a source of focussing error.

 

[pvdhaar]

How would you measure something exactly from the film plane? That's the challenge. Obviously, you don't want to touch the shutter curtain, which would mean that you would want to have the camera on a tripod, depress the button fixed on B, have a meter rule touch the film back and place an object at the end of the meter rule.

Most cameras have a film plane indication stamped into or printed onto the top deck. It's a small symbol of a circle and a horizontal line through it.

 

[sf]

well, the DOF markings on the lenses are not always accurate. That would make that test difficult to measure.

 

[pvdhaar]

well, the DOF markings on the lenses are not always accurate. That would make that test difficult to measure.

The DOF markings are indeed dependent on that the manufacturer thinks is acceptable as a circle of confusion. In practice, you'll find that this ranges from 0.033 mm to 0.0275 mm for 35mm format lenses.

But we're not talking DOF markings here, instead it's the distance marking that has to be used. It's dead center in the middle of the DOF scale.

 

[waileong]

Leica M

Leica M

We're talking Leica M's here. I see nothing on my M4 top plate, and I see nothing on the base plate either.

Most cameras have a film plane indication stamped into or printed onto the top deck. It's a small symbol of a circle and a horizontal line through it.

 

[pvdhaar]

We're talking Leica M's here. I see nothing on my M4 top plate, and I see nothing on the base plate either.

Ok, if it's not marked on the body, it's 2.8 cm behind the front of the bayonet mount for the Leica M series.

 

[alice]

thanks all for your suggestion, i tried to measure something from 1m, and then use a measuring tape to check if it is really 1m from the camera (2.8cm from the mount)....
it seems to match but i don't know how much a few mm off will affect the results because its kinda hard to measure

 

[daveozzz]

My experience (with CV lenses and bodies anyway) is that the distance scales aren't too accurate.
Best way I find is to ignore all measurements and turn the body into an SLR as mentioned above.

Mount it on a tripod and point it at something roughtly 1m away.
Get a cable release that locks and in B mode open the shutter so that it stays open without you having to keep a finger on the shutter.
Open the back and put a small piece of Scotch tape (the frosted white type) over where the film would normally be - you then can see what's in focus (very accurately if you use a 8x loupe for instance).
Focus on the close object with the rangefinder.
Check what the actual focus is on the Scotch tape - if you have to readjust to make it sharp then your rangefinder is out.

Make sure you remove the tape BEFORE releasing the shutter!

 

[daveozzz]

Em, sorry - dudn't realise I was resurrecting such an old post! I'm sure it's all sorted by now - as you were.

 

[former member]

This has come up before, but this thread is fresh.

This is probably the real reason why the Nikon RF's and Canon 7 have "T" settings which hold the shutter open until you turn the shutter speed dial away from "T".

This thread on the J-3 repair shows how I've been setting the shims to get the focus, RF, and distance scales to all agree:

http://www.rangefinderforum.com/forums/showthread.php?t=13670

You can have the RF and Distance scale agree, but if the lens is not sitting correctly in its barrel, the focus will be off. DOF at F2 should be more than 1" at 36", and the RF is more accurate than that. One quick suggestion, infinity on an RF can be ~300ft. If you have it calibrated for 1mile, the close focus "could" be off.