typical close focus accuracy

[cwoodhou]

I'm sure this has been discussed before but I could do with your collective wisdom.

My M6 has just come back from Milton Keynes and the focussing has been adjusted. I checked it out at lens-infinity on very distant objects with 4 lenses. No problem. I checked it with the 90 Elmarit M at its closest focus, with a viewfinder magnifier and a focus checking chart (basically a ruler at 45 degrees from horizontal). Loaded with film, at 70cm, the focus is consistently 1.4cm out. That's about 2%. I quite like the idea of doing portraits with very selective focus, otherwise I'm not bothered.

Would you call this result typical, good or poor?

regards
Chris Woodhouse ARPS
www.waybeyondmonochrome.com

 

[Sonnar Brian]

I would call that result typical of an RF, and it is not so easy to keep the RF and lenses adjusted. On top of that, you get focus shift as you stop certain lenses down. So the question becomes how best to optimize a camera and lens. If this is the combo that you plan on using most often, or at least is most critical, send the camera and lens to the repairshop to have them adjusted for each other.

This 1953 J-3 is shimmed for my newly acquired M8.

Perfect alignment is for close-up and the lens set at F2. At F4, it back-focusses slightly. At F1.5, shown here, it is within the DOF.

At F1.5:

At F4: slight back-focus from focus-shift.

 

[ferider]

Not sure how you get to 70cm with the 90 Elmarit.

But 1.4cm at 70cm is within the accuracy of the M6 RF, also with the magnifier. I have corrected small errors like this (for my 75/1.4, for instance) by adjusting the camera's RF cam a teeny bit. The adjustment is so small that before and after (and using a magnifier) the patch will look like it's perfectly overlaping. Just the "snap" changes a little. Hard to describe in words really.

Also, this might be trivial, but allow me to say it anyways: if the ruler shot at 45 degrees shows 1.4cm off, the focus error is really 1.0cm.

Roland.

 

[cwoodhou]

Not sure how you get to 70cm with the 90 Elmarit.

But 1.4cm at 70cm is within the accuracy of the M6 RF, also with the magnifier. I have corrected small errors like this (for my 75/1.4, for instance) by adjusting the camera's RF cam a teeny bit. The adjustment is so small that before and after (and using a magnifier) the patch will look like it's perfectly overlaping. Just the "snap" changes a little. Hard to describe in words really.

Also, this might be trivial, but allow me to say it anyways: if the ruler shot at 45 degrees shows 1.4cm off, the focus error is really 1.0cm.

Roland.

Err - yes - my mistake, it was 1m on the 90mm. I was confusing it with the min focusing of the 50. I already factored in the 45 degree slop and the error along the optical CL was 1.4 cm. The percentage error will be 1.4% or a focal plane error of ~1.4 mm - so it cannot be a film flatness issue. I'll just make sure to give plenty of camera shake and nobody will be able to detect the focus error.🙂