[Leica] Zeiss Ikon Focus Shift

[billgem at hotmail.com (Bill Marshall)]

On Monday, Frank Dernie wrote:
>"I think thre would be a big difference between floating elements to 
>correct aberrations at different focus distances and a mechanism for focus 
>shift with aperture, which would require some sort of linkage between the 
>aperture ring and the focus ring - not something I've heard of before."

You are quite correct, Frank. To end the speculation, I e-mailed Zeiss Ikon 
with the question. Here is their reply:

"No mechanical coupling between focus & aperture mechanisms. The (lens) 
design is just very well corrected for spherical aberrations."

Since focus shift is particularly a problem with ultra-wide angle lenses & 
fast telephoto lenses shot wide open, I assume that it is the 85/2 & the 
15/2.8 were the lenses of greatest concern & that is why they received 
special attention to their lens designs with floating elements, the 
selection of special glass, & the non-linear RF mechanism built into the 
85/2.

Focus shift is a real enough phenomenon, resulting in softening of 
resolution, which may not be recognized as "focus shift", but which can be 
seen in images that are not quite as sharp as they could be. Some folks 
regard Zeiss's marketing comments on this to be hype because it has not been 
a problem with Leica lenses except for the Noctilux (see E. Puts), but that 
may be more a reflection of just how the high quality of Leica lenses is 
taken for granted.

Maybe Zeiss should just have said that there lenses are very sharp & left it 
at that. Maybe they should not have gotten technical. OTOH, this may be 
Zeiss's way of saying "We're as good as the best; we've attended to every 
detail." I think that some have interpreted it as their way of saying that 
they're better than the rest - including lLeica.

The fact that Leica has only had a focus shift problem with the Noctilux 
does not mean that this problem only occurs on ultra-high speed lenses. It 
just means that this is the only lens for which Leica has not yet completely 
solved the problem. And it means that they don't offer a 15 mm lens at any 
speed. Critics who have said that Zeiss shouldn't be mentioning this because 
they don't offer any ultra-high speed lenses again miss the point that it 
had to be addressed in the 15/2.8 & in order to be successful with the rest 
of their lens line, Zeiss had to address the problem with other lenses like 
the 85/2 as effectively as Leica has. By doing this has Zeiss developed a 
leg up on the rest of the competition - meaning Cosina Voigtlander? Who 
knows? But by offering 15/2.8 & an 85/2, they do have a leg up on CV if 
these lenses are of high quality because CV doesn't offer anything as fast 
in either of these focal lengths.

Ultimtely these questions will be answered by users' experience & by product 
evaluators. They will just want to know how well the lenses perform & if 
they're sharp. If the lenses measure up, no one will care about the hype or 
the tech-speak that was used to promote & explain these new products.