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 (Marty Deveney)
Date: Sun Dec 14 16:14:59 2008

Len wrote:
>The two lenses that I have that have a definite focus shift are  
>the Noctilux and the 75mm Summilux.

Apologies if I jumped too fast, but there is some incredible garbage being 
promulgated on the web about focus shift.

The f1 Noctilux and the 75 Summilux definitely display the most focus shift 
of the Leica lenses designed after 1970 or so (I haven't tested many earlier 
lenses, but they tend to be more conservative designs so they may not show 
much shift).  As they left the factory, the 75s were adjusted to have a tiny 
amount of front focus at f/1.4.  As you stop down, the focus shifts backward 
but within the depth of field.  If you adjust for perfect focus at f/1.4, 
your lens will focus incorrectly until depth of field catches up at f/8.  
Erwin describes the behaviour of the Noctilux and its focus shift here:

Dante Stella gives a good explanation about why adjusting the rangefinder in 
your camera to 'fix' focus shift is hazardous: if you have a range of 
fast lenses, don't even think about it.

My 35/1.4 ASPH tended to focus behind the point of focus when I first put it 
on my M8.  A trip to Leica fixed that.  It displays remarkably little focus 
shift considering the design - perhaps Leica will redesign all their lenses 
to include a floating element at some stage.



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