--- Bill Stanke <bstanke@xxxxxxxxxxxx> wrote:
> It was always my understanding that a lens designer
> would want to use as
> *few* elements in a design as possible.
>
> In the days before lenses coating, the number of
> elements was crucial, since
> each additional element resulted in less contrast,
> and less transmission of
> light. Lens coating technology has greatly reduced
> the drawback of adding
> an additional lens element to a design. However, I
> always thought that one
> reason to use an aspherical element was to *reduce*
> the number of elements
> in a design.
>
> In my Leica Manual (circa 1972) it describes the
> design of the (two
> element!) 560 mm f/5.6 Telyt as follows:
>
> "A cemented doublet telescope objective gives
> excellent definition over a
> semi-field of 2 or 3 degrees; therefore this simple
> system can be used in
> lenses of 400 mm or longer focal length for a 35 mm
> camera. The length of
> the tube from lens to film is, of course equal to
> the focal length.
> However, the lens is very light in weight and gives
> excellent definition at
> apertures of f/5.6 or smaller."
>
> Granted, this is a longer focal length, but if E.
> Leitz thought two elements
> were the right number, then I won't quarrel with
> them.
>
> I think the only useful test is to go out and shoot
> the lens in the
> situations where a focal length of 200 mm is useful.
> Try and minimize
> camera shake as much as possible in the situation
> (tripod, monopod, bean
> bags, self-timer etc.) If you're happy with the
> results, don't worry about
> the "bad" test results.
>
> Bill Stanke
Amen to the last part....however E. Leitz
notwithstanding, a cemented or even air-spaced doublet
would almost have to show some color fringing at high
enlargments, you know, that violet fringe at the
border between light or white objects against a dark
background. And I think that with longer focal lengths
the distance between where different colors are
brought into focus is increases. If I remember
correctly it takes 3 or 4 elements to bring 3 colors
in the visual spectrum into focus at the same point.
With all the advances in lens design in the last 20
years I would be surprised if the current R lenses
include any long focus designs. John Robison
__________________________________________________
Do You Yahoo!?
Yahoo! - Official partner of 2002 FIFA World Cup
http://fifaworldcup.yahoo.com
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|