[Leica] Zeiss Ikon Focus Shift

[billgem at hotmail.com (Bill Marshall)]

The topic of focus shift in relation to the Zeiss Ikon was raised over the 
weekend. So, I did some research & came up with the following explanation 
from Zeiss Ikon:

"One major challenge for all lens designs is to correct for spherical 
aberrations. The larger the largest opening is, obviously the more problems 
to correct over the entire aperture range. f/2-f/2.8 lenses have always a 
potential focus shift which can cause problems.

"Normally the approach is to use the widest opening as the "correct focus" 
position and accept that when stopping down the position of best focus is 
changing (also the depth-of-field is increasing). However, for longer lenses 
this can be recognized as a visible shift of best focusing point. The 
typical situation is for a portrait when you know that you made your 
focusing on the eye, but the sharpest parts in the picture are on the ears. 
Irritating even if the eye is fairly sharp. But depth-of-field is not the 
same as very good sharpness.

"The ZI lenses have been designed for eliminating this effect. The result is 
an even higher performance at stopped-down apertures, where you normally use 
the lens. At full aperture (where lenses are normally tested & judged) you 
would see a smaller difference between designs with & without significant 
aperture dependant focus shift."

So, it would seem that the primary beneficiary of this correction will be 
the 85/2 Sonnar, which I would expect that purchasers of this lens will 
appreciate since it is the highest priced pony in the stable. At the 
announced price, this clientele would more likely be professionals involved 
in portraiture for whom such critical differences are more likely to be a 
factor.