[Leica] Focus, Noctilux, M8, C Mode & Scotch

[imagist3 at mac.com (Lottermoser George)]

The view finder is parallax corrected. The range finder windows sit  
above the lens (when camera is horizontal) and never truly relate to  
the center of the lens except as to the relationship between the film  
plane (chip plane) and the plane of focus.

Here's what Joe Englander had to say on Leica Camera User list:
entire thread at <http://www.leica-camera-user.com/digital-forum/ 
18053-testing-focusing-accuracy.html?highlight=focus+test>

I haven't gone through all the posts about the M rangefinder and its  
lenses focusing inaccurately, but I have read enough of them-- 
especially regarding the 35/1.4 ASPH-- and of the techniques used to  
come to their author's conclusions, that I am motivated to point out  
what looks like specious technique. The conclusions and their  
explanations may be correct but since the method seems suspect,  
correct conclusions would be more by accident than accuracy. I am not  
saying that M lenses are not sometimes in need of adjustment, nor  
that M rangefinders do not go out of whack, nor that there are not  
focus shifts inherent in various designs, only that one of the most  
popular techniques for testing focusing accuracy is probably not  
appropriate.

In order to test a precision apparatus, there ought to be an  
appropriate, precise and repeatable testing technique. Using a target  
stretching away from a tilted camera is not such a technique for  
rangefinder cameras, especially if you want to be precise. Such a  
method may be appropriate for testing focusing through the lens but  
the rangefinder itself is neither at the same height nor at the same  
position as the lens being tested.

The finder window is above and to the side; this is obvious, but its  
importance for testing focus seems to be overlooked. Distances are  
calculated from the finder window and it is not centered above the  
lens. The M compensates framing for parallax but does not shift the  
focusing mechanics in the same way for closer subjects. If the camera  
is tilted 45 degrees toward a ruler, the rangefinder itself is now  
CLOSER by a measurable amount to the target than the center of the  
sensor. This difference in distance may be enough to influence any  
critical testing of focusing. The rangefinder cannot be accurate both  
for its position and for a position that is behind it, eg, the center  
of the sensor. When people complain about 10 or 20 mm of inaccuracy  
and I can see that the rangefinder of a titled camera is that much  
forward of the imaging plane, it seems likely that the angle and not  
the rangefinder is the problem.

The same phenomenon would hold for tilting the target where one side  
of the target is closer to one side of the camera than the other: the  
viewing & range finding window is not the same distance from the  
target as the lens and the center of its sensor.

The only technique I know that avoids this problem and that is  
available to users for testing the near-distance focusing accuracy of  
M lenses and rangefinders is to have a target that is absolutely  
parallel to the imaging plane, to focus on that target and then to  
change the distance between the target and the camera by small  
distances without changing the lens' focus. Evaluation of the  
accuracy of focus is based on comparison of the resulting images.

The best method I have found for assuring absolute parallelism--and  
it must be absolute, not "sort of"--is to use two mirrors, one  
attached to the camera lens and one attached to the target, one of  
which has a hole in it for viewing (with rangefinders this one is on  
the target). While peering through the mirror attached to the target,  
the target is adjusted until an infinite reflection is seen in the  
tripod-mounted camera's mirror.

Either the target or the camera can then be moved closer or farther.  
But since parallelism must be maintained, I have found that mounting  
the camera on a micrometer macro focusing rail is easiest. The camera  
is then moved back and forth in increments as small as you like. Your  
lens and camera may be best at a zero position or it may be better  
closer or farther from the target. There may be shifts of focus based  
on aperture or they may not be appreciable, after all. Ultimately, I  
have always been amazed at how accurate the Leica equipment is and  
how often I have found that any focusing inaccuracies were frequently  
due to operator error.

When there have been inaccuracies, I have been much more confident of  
my conclusions using this technique because it is repeatable and the  
results are comparable. I can compare my results with others'; with  
the tilted camera method, that is unlikely and probably explains a  
lot of the variation in forum member's results. If I get different  
results at different times, the problem is most likely my technique  
similar to making pronouncements after miss-focusing in the field. If  
the best results are -10 mm on the first and subsequent iterations  
with one lens but 0 with another, I am certain the problem is the  
lens and can evaluate whether I think that at 1 meter I will ever be  
able to focus within that error while both my subject and I are  
breathing. Clearly, if it is -20mm with all lenses, there is a  
problem with the camera. I always hope to find it is my error because  
it is easier for me to correct my behavior than to send my equipment  
away.

Joe
--------------------------------

Regards,
George Lottermoser
george@imagist.com



On Mar 19, 2007, at 12:05 PM, Carl Muckenhirn wrote:

> Isn't the rangefinder supposed to indicate the "focus" point of the  
> lens? Isn't that why it is parallax corrected?