[Leica] Re:

["Henning J. Wulff" <henningw@archiphoto.com>]

>Is there someone who can explain me what is a floating element, in optic.
> 
>thank you
> 
Gregoire Vandenschrick,   Assistant de recherche


'Floating elements' are elements of a lens that move independently of the
other lens elements to correct for aberrations when focussing at close
distances. Most 'normal' lenses are corrected for best performance down to
10x the focal length, or so, and do not perform as well at closer
distances. The problems also increase with strong retrofocus designs and
high speed lenses. The first lens to have this feature was the first Nikkor
24/2.8, which came out in the late 60's. Now it's used in many wideangles
for SLR's. I don't believe any M lens uses this technology, mostly because
they don't focus that close, and also because they are not at all or only
slightly retrofocus.

Macro lenses use this technology frequently as well. 50-60mm lenses used
this for aberration correction initially, but now they as well as longer
macro lenses use this for reduction in physical length as well. That's why
a 105mm macro lens can get down to 1:1 without being racked out an
additional 105mm, and why 200mm macro lenses are possible at all. They use
'floating' elements to change the focal length as well as correct
aberrations, so that at 1:2 a 200mm macro actually only has a focal length
of 140mm or so. 'Floating elements', 'Internal focus' and 'zoom lens
groups' all do the same thing: move with respect to the rest of the lenses,
only they do it for different reasons.
   *            Henning J. Wulff
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