Archived posting to the Leica Users Group, 2008/12/13

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Subject: Re. [Leica] M8 upgrade - 90 Cron APO focusing problems now Question????
From: pklein at (Peter Klein)
Date: Sat Dec 13 19:00:08 2008

In my previous post, I put in an ASCII diagram illustrating focus tolerance 
on film vs. a sensor, and messed it up.  Here is a screen clip which will 
show things correctly:


I wrote:

>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 
>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 
>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.

(Diagram deleted, see here instead):

>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.