Measuring Focus Distance on a RF

[kemal_mumcu]

This is sorta a newbie question. I feel like I should know the answer but I don't think I've seen this addressed before:

From where on the camera do you measure focus accuracy? I have a Leica M2 and I'm not sure about how well its focusing around the 2 - 3 meter range. I want to set up a test sheet and measure the distance between it and the camera but from where; the film plane, the flange, or the end of the lens? My guess is the film plane but I wanted to ask here for a second opinion.

Thanks

 

[Roger Hicks]

Film plane. You're right.

Cheers,

R.

 

[kemal_mumcu]

Thanks. 🙂

 

[Steve M.]

You might try using the old large format trick if this would work w/ an M2. Open the back, and carefully and tightly and run a couple of pieces of scotch magic tape (the frosted kind) across the back of the film plane. Mount the camera on a tripod and hold the shutter open on B (easier w/ a locking cable release). Focus on something you've marked off at a measured distance by looking at the image on the tape. I use a slide loupe to help w/ this. Once you have the image focused as good as it will get ck your RF double image and see if it agrees. If it doesn't adjust your RF. Usually checking it at infinity will make everything right, but to be sure it's best to ck it at infinity and about 6'.

I always do this w/ my old MF folders and they take much better pictures than most people think they're capable of.

 

[payasam]

I check range-finders by taking photos, at closest focus, of a ruler placed length-wise to the camera, which is on a tripod and looking down at around a 30-degree angle. This is good if, like me, you want accurate focus at near distances with 50mm and longer lenses. You could of course use ground glass or Magic Transparent tape at the film plane instead of taking pictures.

The infinity adjustment on an M Leica is easy to do, and it is this which is ordinarily needed. I was never able to get the hang of the 1-metre adjustment, but I have read of several ordinary users who did it.

 

[ampguy]

The problem with rulers is your angle, unless measured and calculated for, will not be to scale.

Sonic measuring devices, with laser pointers for specifying target distance work fast and quickly for me. For critical work, I will re-measure with multiple tape measure methods.

For a right triangle, you can use this calculation: c2 = a2 + b2

But for an oblique triangle, you need to use this: c2 = a2 + b2 – 2ab cos C

If you do focusing tests with a flat ruler at an angle, you'll soon realize that the markings on the ruler are meaningless. For example if you measure down at a ruler at "4" and the range of 2 to 6 inches, cm, or whatever are in focus, it doesn't mean your DOF range is 4" until you correct with the above trig.

 

[payasam]

You are correct, ampguy, but I have found that my R/F adjustments are good enough even without such precision. The markings on the ruler have enough meaning for my needs. I deal with centimetres, not with millimetres or microns.