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

[carlmuck at verizon.net (Carl Muckenhirn)]

Given the distance from the center of the lens mount to center of the  
RF patch is ~54mm, at 45 deg. the rf patch is ~35mm closer to the  
target. If the DOF is less than that then, assuming accurate focus,  
the difference is visible.

So simple fix is to rotate the camera (and if we're getting  
persnickety, make sure the focus point is on a line 1/2 between the  
center of the lens and the center of the rf patch) then you can test,  
informally to your hearts content.

Most of us do not have an optical bench to play with, so these  
informal tests are all we've got!

c.

On Mar 19, 2007, at 1:17 PM, Lottermoser George wrote:

> 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?
>
>
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