Re. [Leica] M8 upgrade - 90 Cron APO focusing problems now Question????

[pklein at threshinc.com (Peter Klein)]

Aram:  As best I understand it, the M8 focus issues occur because film has 
thickness, but the sensor is, for all practical purposes a flat 
plane.  (Yes, I know it has wells and microlenses, but the bottoms of the 
sensor wells are all in the same plane).

When light hits film, as long as it comes to its exact point of focus 
somewhere within the sensitive emulsion, it's going to zap a group of 
silver halide molecules, and you will have a sharp image.  When light hits 
the sensor, it must be in focus exactly at the sensor surface, plus or 
minus what our eyes can detect at a given print or screen viewing size.

So let's take a ruler and let it stand for a greatly magnified film 
emulsion, viewed on edge.  Let's say that in this scale of things, the 
emulsion is two inches thick.  The lens is at the end of the ruler, 
projecting back towards zero.  The lens can be adjusted so that our image 
comes to sharpest focus anywhere between the 0 and 2 inch mark, and all's 
well.

But with the sensor, we have much less tolerance.  The light rays must come 
to focus between 1.75 and 2.25 inches, or it will look out of focus.

Now let's say that the lens is adjusted to come to focus at the 0.5 inch 
mark.  It will look fine on film.  But it will be out-of-focus on the sensor.

NOTE:  The diagram below will only look correct with a monospaced 
font.  The distance between the vertical bars on each line must be the 
same. If not, copy it into a text editor, or use Courier font in a word 
processor.  The x's represent the zone where best focus is possible with 
each medium, the o's represent where the image will appear out of focus.

|xxxxxxxxxxxxxxxxxxxx|  (film)
|oooooooxxxxxoooooooo|  (sensor)
|     1                            |  (point of best focus #1)
|                2                 |  (point of best focus #2)
|                    3             |  (point of best focus #3)

If a lens is adjusted so that it comes into sharpest focus at Point of best 
focus #1 above, it will give in focus images with film, but out-of-focus 
images with a sensor.  A lens adjusted to Point #2 will appear in focus on 
both film and a sensor.  And a lens adjusted to Point #3 will look fine on 
film, but will have all its depth of field to one side of the focus point.

This also shows why focus shift may show up on the M8 when we never saw it 
with film.  Let's say the lens wide open brings the image to perfect focus 
at Point #1, Point #2 at one stop down, and Point #1 two stops down.

--Peter

At 11:38 AM 12/13/2008 -0800, Aram wrote:
>Question.  I am not an M user, but am curious.  How can this be?  How can a
>lens focus properly on a film camera but not on the digital M?  If this
>happened with every lens, I could understand that - something out of whack
>with the camera.  But if other lenses that worked on a film camera also work
>on the digital M, why would one lens behave this way?