Archived posting to the Leica Users Group, 2005/03/14

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Subject: [Leica] Leica Glow
From: rpalmier at depaul.edu (Bob Palmieri)
Date: Mon Mar 14 07:16:54 2005

BD wrote (in response to a post about the qualities of the 35 
pre-asph.):

ROFLOL! :-)

Glow=flare, distortion,softness

Flare and "softness" can certainly contribute; distortion I'm not so 
sure about, unless one wants to link residual distortion in optical 
designs to tradeoffs in astigmatism, field curvature, and spherical 
aberrations.  Me, I certainly wouldn't go there...

However, let me add some more speculation to this persistent subject 
(I'm sure I'm not the first guy to bring this up.)  Simply put, some 
lens designs image a point of light as a bright spot surrounded by a 
darker patch (Leica lenses are by no means unique in this regard.)  
Areas of the negative which don't get much exposure don't show this 
dark patch.  Therefore, they show higher resolution.  The highlight 
areas do show up this additional blurring component, therefore edges 
get softer in the highlight areas.

So, it is possible for a lens to look softer in the highlights than in 
the shadows.  Some lenses exhibit this quality more than others.  Throw 
in a bit of flare, etc., and there ya go...

Bob Palmieri