Archived posting to the Leica Users Group, 2008/12/14

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Subject: [Leica] M8 upgrade - 90 Cron APO focusing problems now Question????
From: freakscene at weirdness.com (Marty Deveney)
Date: Sun Dec 14 20:41:40 2008

Steve Barbour asked:
is this simply due to the fact that with their potentially very narrow? 
dof? they will show any focus shift? most readily? ?

Partly but not entirely.  The actual shift is caused by the degree to which 
spherical aberrations are controlled.  These lenses have a lot of focus 
shift because they typically display a lot of spherical aberration (for a 
Leica lens).  You can see it more easily because the depth-of-field (DoF) is 
sufficiently narrow that stopping down does not increase DoF more than the 
shift.  With the Noctilux, for instance, even stopping down from f2.8 to f4 
shifts the focus more than the increase in DoF, despite the DoF at these 
apertures being fairly large.  This occurs because the design permits a lot 
of residual spherical abberration.  The new f0.95 Nocti does not display 
this tendency - the aspherical and floating elements decrease spherical 
aberrations to the point where focus shifts less than DoF changes.

Len wrote:
It  has been my feeling that the older lenses were either built to  
tighter tolerances or tested to more stringent standards. I have no  
way to verify this except for my own experience and testing.

In measuring the performance of some older Leitz lenses it is apparent that 
manufacturing tolerances have improved enormously, but that individual 
matching of lenses and hand collimation combined with stringent quality 
control assisted greatly in limiting individual variation.  One other 
observation i've made is that focus tends to be in the middle of the DoF in 
older lenses, while newer lenses tend to put a third in front and two thirds 
behind the point of focus.  Most older lenses have designs that are 
sufficiently conservative that DoF is greater than focus shift (it certainly 
is with all the 35 and 50 mm Summicrons), although the various fast 50 mm 
Sonnars are subject to focus shift.  I've never used an 85 mm Summarex.  It 
wouldn't surprise me if some older really fast lenses don't resolve enough 
at wide apertures for a variety of reasons to show the shift.  

Marty

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


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