[Leica] Zeiss Ikon Focus Shift

[henningw at archiphoto.com (Henning Wulff)]

At 4:23 PM +0000 1/10/05, Alex Hurst wrote:
>Bill wrote in part:
>
>>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.
>>
>
>FWIW, this would seem to be a variation on the Nikon CRC system 
>which optimises focusing in close. My 85/1.4 Nikkor was apparently 
>one of the first short teles to have it. Nikon also use it on a 
>number of other fast lenses.
>
>Best
>
>Alex
>--
>Alex Hurst
>Waterfall
>Nr. Cork
>Ireland

The Nikon CRC system (otherwise known as 'floating elements') is 
aimed at correcting other problems than focus shift. The floating 
elements stay in the same relative positions when the aperture is 
changed, but move when the focussing distance is changed. The 
floating element feature corrects for the fact that all lens 
configurations are at their optimum at one reproduction ration only, 
and performance falls off at others.

The focus shift issue is mostly due to spherical aberration, as has 
been noted in a previous post. Current ASPH Leica lenses have about 
as little of this as any lenses of their focal length and general 
design, so it is not an issue anymore. The 90AA would be very hard to 
match, let alone beat in this regard. I would be surprised if the 
85/2 Zeiss lens performs better, and if it does, whether anyone can 
tell. :-) The 75/1.4 has _very_ slight focus shift; certainly less 
than any tolerances introduced in a practical shooting situation. It 
has more undercorrected spherical aberration, and thus also nicer 
bokeh. The older Summicron 90's had more focus shift (and spherical 
aberration), but this was somewhat hard to tell as they were not 
super sharp wide open, and so the exact focus point was often hard to 
tell.

-- 
    *            Henning J. Wulff
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