Body focus comparison test between the Hexar RF and Leica MP

[awilder]

To make a long story short, my body focus comparison testing revealed agreement to well under 0.0175 mm. This is even better than the current Leica tolerance of 0.02 mm. Testing was done using a 21/4.5 ZM Biogon and examining the negs under a 40x microscope. The subject was a USAF resolution chart obtaining lens resolution of 100 lp/mm with either body when the lens was set for the exact same distance. Not only is this is an indication of body focus accuracy but film plane focus is also spot on. Body focus agreement is much more critical with shorter focal length lenses than longer ones due to the very short distance the lens barrel travels through it's focus range.

 

[mfunnell]

Not surprising, but nice to know. Thanks for passing on the results.

...Mike

 

[wintoid]

Body focus agreement is much more critical with shorter focal length lenses than longer ones and the Hexar came through splendedly.

Really? My "common sense" bone would have said the opposite, but I guess it's wrong (again!). You learn something new every day.

 

[awilder]

What is very critical on short focal length lenses is the correct distance to the film plane so the lens' back focus matches the camera manufacturers specs of the film plane focus. Once that is done, focusing accuracy doesn't have to be all that precise because of the great dof. That's why you can scale focus a 25/4 CV Skopar or 15/4.5 CV Heliar with good results.

 

[joachim]

... Body focus agreement is much more critical with shorter focal length lenses than longer ones due to the very short distance the lens through it's focus range.

I am afraid it is not 🙁 The tolerances with respect to the back focus depend on the speed of the lens only. You should redo your test with your fastest lens.

 

[awilder]

The ZM 21/4.5 has extremely high resolving power and therefore even very small changes in the focus setting can make a visible difference when evaluating resolution test charts on a negative with a 40x microcope even at an aperture as slow as f/4.5. Being a slow wide angle also minimizes errors due to focus shift commonly found with most fast lenses. A shift in focus of +/- 1 mm on the distance scale of the focusing ring of this lens will show a difference in the sharpness of the negative due it's very high resolving power. This translates to +/- 0.0175 mm movement of the lens barrel. Regarding high speed lenses, if we assume a very tight dof using a CoC of 0.01 mm, an f/1.5 speed lens requires a body focus accuracy of +/- 0.015 mm to produce a sharp image.

 

[aizan]

"body" focus, not "back" focus, joachim.

 

[CameraQuest]

awilder,

while it is good indeed your Konica RF body has no back focus problems, it only shows YOUR Hexar RF has no back focus problem. This issue can which can vary from Konica RF to Konica RF body.

My own Konica RF had that problem, until it was hand delivered to the factory via a senior Konica consultant and adjusted. At one time it was reported on the net that Konica Europe was adjusting RF bodies for free.

Leica was not at all happy Konica was producing the M mount RF -- the first Leica M mount camera after Leica allowed the patent to expire by paperwork mistake. The Konica consultant mentioned above has personally seen the Leica technical M mount drawings which were provided by Leica per Konica's request. What was not asked for, and not provided, was the plus / minus tolerances of the lens mount and film gate. Konica guessed them wrong. Some bodies were assembled beyond those tolerances, some were not. This created the contradictory "my RF body has back focus problems" and "my RF body is OK" reports.

Stephen