[Leica] Noctilux DOF Film vs M8

[pklein at 2alpha.net (Peter Klein)]

Steve: It has to do with the range of the RF roller adjustment that will 
result in acceptable focus with each lens.  The lenses are calibrated to a 
standard, like the body.  But in the real world, things may not be quite 
right, and the M8 is less forgiving than with film--in other words, the 
tolerances before things go awry are less.

Note: The diagrams below will only look correct if you are using a 
Monospace font like Courier to view your email.

Think of a yardstick and a 1-foot ruler placed on a table, with the ruler 
below the yardstick. Now imagine that the yardstick is the adjustment 
range that work with the 50/2 Summicron, and the ruler is the range that 
will work for the Noctilux at f/1. I've pictured it below. The asterisk 
represents where the RF roller is adjusted. Here is how your wide-open 
Noct and M8 may been set, before you adjusted the camera:


(Fig. 1)
|SummicronSummicronSummicron|
        |Noctilux|
      *


As adjusted above, the camera will focus properly with the Summicron, but 
not with the wide-open Noct.  However, we could adjust it so that the 
focus is OK with the Noct, and still be OK with the Summicron. Below is 
what you probably did:


(Fig. 2.)
|SummicronSummicronSummicron|
        |Noctilux|
            *


The problem with the Noct is that if you stop it down 2 or 3 stops, you 
get something like this:


(Fig. 3)
|SummicronSummicronSummicron|
                                       |NoctiluxNoct|
            *


Now there is no way to get the Noct exactly right without messing up your 
other lenses like the Summicron.

The Summicron also focus shifts a little bit when you stop down 2 or 3 
stops, but nowhere near as much as the Noct.  So you might get the typical 
M8 "all depth of field in back of the point of focus," but things are 
still usable with the Summicron. Not with the Noct.


(Fig. 4)
           |SummicronSummicronSummicronSummicron|
                                       |NoctiluxNoct|
            *



On film, the shift is less than with either lens.  The problem still 
exists, but it is within tolerance for the Summicron, and still 
visible (though less) for the Noct:


(Fig. 5)
    |SummicronSummicronSummicronSummicron|
                               |NoctiluxNoct|
                 *


Does this make sense?  In real life, the initial position of the RF roller 
adjustment may vary, (asterisk) as may the calibration or the individual 
lenses (sideways position of the "ruler" or "yardstick."

The point is that there is no absolute "perfect" point of focus.  There is 
only a range of acceptable tolerances.  With the M8, those tolerances are 
noticably smaller.  It also appears that in order to get some lenses to 
focus correctly wide-open, the RF and lens are adjusted so that most of 
the DOF is behind the point of focus (Summicron in Fig. 4), rather than 
the classic 1/3:2/3 distribution most of us learned in the film era (Fig. 
5).

I'm sure the folks at Leica knew all this for a long time. It's just that 
in the film era, most of us didn't notice.  The M8, with its instant 
feedback and stricter requirements, opened our eyes.  The confusion exists 
because the above is not intuitive--most of us thing as focus as being 
either correct or not, and we don't think about focus shift.  The 
situation has been made worse by quality control problems, used lenses 
that were out of adjustment, and perhaps by Leica initially adjusting 
lenses to a standard that was viable for film but insufficient for the M8.

I don't understand if or how focus shift also varies with respect to 
subject distance. No one I've ever asked seems to understand it, either. 
Sometimes DOF seems to cover focus shift at longer distances. Some people 
have observed that focus shift is always the same amount of offset "twist" 
from what the RF tells us, so it may be in proportion to the subject 
distance, ie. linear with respect to the focus cam.  This may break down 
at sufficiently large stops or long focal lengths.

--Peter

Steve Barbour wrote:

> a wonderful review and summary Peter...

> what I really don't understand is the following two points and their
> (apparant) incongruity...
> re fixing "back focus"....

> we either change the set screw in the body with an Allen wrench, OR we
> send our lenses somewhere, WITHOUT the body, to be adjusted...

> if you can fix the problem by changing something in the body, how can
> fixing the lenses with NO regard to the settings/status of the body
> possibly work?