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

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Subject: [Leica] M8 upgrade - 90 Cron APO focusing problems now Question????
From: freakscene at weirdness.com (Marty Deveney)
Date: Sat Dec 13 16:35:02 2008

>But every Leica lens I have tested has a focus shift when going from film 
>to digital.

This is not focus shift.  This is a difference in body-to-lens calibration.

There are a few interrelated issues here.  Focus on a rangefinder works by 
mechanical linkages.  The camera focuses correctly as long as a certain 
level of mechanical precision is achieved and individual lenses and bodies 
are within spec.

Focus shift is a property of lenses caused by uncontrolled spherical 
aberration - it has nothing to do with camera bodies.  Think of depth of 
field as a theoretical band of acceptable sharpness in space that varies in 
width and with distance from the film or sensor plane as you stop down. 
Acceptable sharpness is related to the circle of confusion concept, which 
ultimately is also linked to human visual acuity, so whether something is 
"sharp" or "in focus" is ultimately also linked to your own eyesight. 

At close range and wide open the focus point of fast RF lenses is almost 
always ever-so-slightly in front or behind the point at which you focus - if 
the lens is properly adjusted.  If the lens or body are poorly adjusted the 
result in softness, but this is 'out of specification collimation', not 
'focus shift'.  As you stop down the focus point shifts away or towards the 
film or sensor plane (depending on the lens' design).  You run into problems 
when this shift is greater than the increase in depth-of-field. I haven't 
tested the 90/2 AA extensively, but the f1 Noctilux, for example, focuses 
close to correctly at f1, but depth-of-field doesn't catch up with focus 
shift until f5.6. This is especially apparent on the M8. The answer for me 
is simple - use the Nocti on the M8 at only f1 or f5.6 or larger.

Why does digital make it worse?  Remember that focus shift works backwards 
aswell as forwards.  Even the thinnest films have an emulsion that is ~120 
micrometres thick or so.  When the virtual image shifts, you have some 
leeway for movement since the image will appear sharp if the image is in 
focus anywhere in the emulsion.  Digital sensors have no depth and the 
photosites have a single point at which focus must be accurate, which is 
almost a theoretical plane is space although in practice it has some depth, 
though this is only a few nanometres.

Some of Len's lenses do display focus shift, but having them calibrated to 
his M8 will help enormously, particularly with the 50/1.4 asph which has 
negligible focus shift courtesy of its design which includes an aspherical 
element _and_ a floating element.  These minimise spherical aberration and 
therefore focus shift.  Of course some of the others will display focus 
shift, but having them calibrated to the body with make sure that they are 
within spec wide open, which has nothing to do with focus shift.

Howard's 90/AA may be displaying a lack of calibration, displaying some 
focus shift - since its design is very simple and the lens is not known to 
be a great performer at short distances anyway.  I suggest, if you want to 
try to make it work, to send the body _and_ the lens to a competent repair 
person for matching and then very carefully testing what is happening 
afterwards to check for focus shift.  If this sounds like too much trouble, 
you could trade your 90/2 AA for the 75/2 AA, which has a floating element 
and an asperical element which do a much better job of controlling residual 
aberrations than the 90/2 AA design does.  Hoppy's 75/2 AA worked perfectly 
on five or six M8s that were present at the Melbourne LuG get-together and 
close-up focus was _much_ easier to nail than with my 75/1.4.

Marty

Gallery: http://gallery.leica-users.org/v/freakscene


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Replies: Reply from len-001 at verizon.net (Leonard Taupier) ([Leica] M8 upgrade - 90 Cron APO focusing problems now Question????)