Some lenses focus by moving all the lens elements in and out -- but the
relation between the elements stays the same at all times.
Some lenses focus by moving one or more elements or groups of elements
in and out at a fixed rate -- the relation between the elements
otherwise remains constant.
With a "floating" group, in addition to the above alteration of the
relationships between lens elements and/or groups, there is one or more
additional group(s) which is/are shifted independently, and at a
different rate of change, during focus or zooming.
On many of the finer Olympus fixed focal length lenses, for example, the
whole lens body moves in and out in relation to the film as the focusing
ring is turned, but in addition the rear lens groups are adjusted very
slightly in and out in relation to the rest of the lens elements.
Zoom lenses will often have several "floating" groups which move at
different rates and times from one another. For example, only the front
lens group of the 75-150mm will move for focusing, and at a constant
rate of change. But when zooming, the 2nd and 3rd groups will move
independent of each other during zooming. The 2nd group shifts at a
relatively constant rate during zooming, while the 3rd group will move
first in, toward the film, then out.
Later, more sophisticated zooms (on the IS's for example) move several
groups in and out at different times during the zoom -- it can get very
busy in there!
M. Royer wrote:
> I know that Olympus uses a 'floating mechanism' or a
> 'floating element' to correct for abberations at close
> distance focusing which makes it a must for macro
> lenses. But what exactly are these elements, how do
> they 'float' and how does that help correct close
> focusing?
>
> Mak Lloyd
>
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