[Leica] Noctilux M8 back focus update -- Erwin Puts inroduction

[tomschofield at comcast.net (Tom Schofield)]

Hi Eric,

We should introduce you to the work of a former LUG member-turned- 
author of the Leica Lens Compendium, Erwin Puts.  You can spend many  
hours on his website: www.imx.nl

Here is an excerpt discussing the focus shift with the Noctilux  
(tables did not paste well) In a nutshell, as you stop down, while  
the focus shift continues, depth of field compensates for it, so that  
it is most problematic at f 2.0, where the focus shift is greater  
than, the depth of field:

"MTF measurements of the Noctilux and the Summicron can be compared  
to give this discussion a more quantifiable dimension.

The Noctilux however has some very special problems in this area.  
Consider the results in the tables below.

% Contrast in center Noctilux-M
lp/mm  1.0   2.0   5.6
10         84   79    90
20         59   41    72
40         27    1     33
The 10lp/mm value at f/1,0 is very good, the 20lp/mm is still quite  
good and the 40lp/mm result is acceptable: the Summicron has the same  
value for the 40lp/mm in the extreme corners. (at f/2,0 mind you!).  
At f/2,0 we notice an overall drop in performance , especially at  
40lp/mm, while at f/5,6 the performance is slightly better than at  
full aperture. Modern Photography tested the Noctilux a number of  
years ago and noticed the same behaviour. They just accepted the  
figures. We know that the Noctilux cannot be fully corrected for  
spherical aberration. One important result of this defect is a slight  
focusshift. (?Blendendifferenz? the Germans call it). When refocused  
the Noctilux showed these results:


% Contrast in center Noctilux-M (refocused)
lp/mm  2.0   5.6
10        94    96
20        80    91
40        51    79

The focusshift of the Noctilux is 74 micron when stopping down from  
1,0 to 2,0 and 120 micron when going to 5.6. The reason that the f/ 
5,6 values do not drop as much as the f/2,0 values (in the first  
series of measurements) is the depth of field. At f/5,6 this depth of  
field is greater than the focusshift, so the results stay within  
tolerances. By the way: 120 microns is the total thickness of a  
typical fine grain 35mm film! These results show the pitfalls when  
testing an extreme aperture lens. As most objects in the real world  
are three dimensional and have depth, the effects of the focusshift  
are hardly noticable. What you could notice is a slight softening of  
the image at f/5,6, because the focusshift will produce a somewhat  
larger diameter of the blur circle."





Enjoy Erwin's site, but keep in mind some of the technical issues he  
discusses are so minute that most people will never realize them in  
ordinary shooting.

Tom







On Apr 24, 2007, at 1:15 PM, Eric Korenman wrote:

> So I spent an evening with my M8, a sturdy tripod and a focus chart.
> I know this is obsessive naval gazing and not art but here is what  
> I found:
>
> The 35mm asph summicron and 4th generation 50mm summicron are dead  
> on at
> near focus and wide open.
> Stepping down aperture shows the focus stays near center.
>
> However- the Noctilux, while dead on at f/1.0 shows severe back  
> focus the
> higher the f stop.
> At f/5.6, the plane of focus is nearly 6 cm behind the target, at  
> target is
> blurred.
>
> I plan to rerun the test with a borrowed M6 and film.
>
> Eric
>
> left calls with Leica NJ - everyone techincal seems to be out of  
> the office!
>
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