[Leica] Re:

["Dan Post" <dwpost@email.msn.com>]

Most times, we think of a compound lens as being composed of a certain
number of elements, mounted in groups, some fixed and cemented together, and
mounted in a tube. The focus is achieved by moving the whole assembly
forward or back in order to focus.
A comnpound lens, with a floating element, is usually a variable focal
length or 'zoom' where there is one or more groups that move independently
from or at a different rate than the other elements when the lens is focused
or the focal length is changed.
In older zoom, the focus was not constant when the focal length was changed-
you set the focal length and then refocused; with a movable element, or
group of elements, the lens could be made to stay in focus while the focal
length changed.
I am sure that the same movable elements are used with the newer aspherical
lenses. Asphericcal lenses to me are the optical equivalent of 'integrated
circuits'! Whereas it was customary to make a compound element of two types
of glass, cemented together but with spherical figures, it has become
possible to make the same element from a single piece of glass that has a
figure that is something other than spherical- such a single lens element
may replace two or three pieces of glass- reducing the weight, size, number
of air to glass surfaces, and absorption of the lens. The result is a
smaller, lighter, more contrasty, and faster lens! Since and aspherical
element or even several my not focus in such a straight forward manner as a
traditional lens- one or more elements in the compound lens may also 'float'
or move independently from the others as the lens is focused at different
distances! I shudder to think what such a lens design would have been like
had it not been for computers! I read that E. Leitz used to hire math
teachers to work on his lens formulae in the Summers, calculating by hand
the ray tracings. It took many of them, working months, together, to get a
reasonable set of calculations to make a lens- calculations that a home
computer could make in seconds today! Designing the mounts and helices, I am
sure, is a monumental task today, even with the help of computers, but the
results- such as reported by Erwin Puts, are nothing short of incredible!
While I look the rendition of the 'old' glass, I am nevertheless amazed and
fascinated by what the imagination of optical engineers can conceive- then
build!
Sorry to prattle on, but I hope this helps!
Dan
- ----- Original Message -----
From: Grégoire Vandenschrick <vandenschrick@geog.ucl.ac.be>
To: Leica User Group <leica-users@mejac.palo-alto.ca.us>
Sent: Monday, June 28, 1999 9:37 AM


Is there someone who can explain me what is a floating element, in optic.

thank you

Gregoire Vandenschrick,   Assistant de recherche
Unite GEOG,  Departement GEO,  Faculte des Sciences
Universite  catholique  de  Louvain

email: vandenschrick@geog.ucl.ac.be
          vandenschrick@page.ucl.ac.be

Tel:    (00 32) 10   47 28 42
Fax:   (00 32) 10   47 25 56
prive: (00 32) 2   762 06 22

Adresse postale:

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