> "Robert Meier" <[EMAIL PROTECTED]> wrote:
> The diopter does only change the focusing distance but
> not the focal length. ...
Hi Robert,
I think your statement applies to extension tubes (which
have no glass to bend light). Diopters bend light and
have to affect the "resultant" focal length of all the
glass in front of the film.
For instance, an EF 100mm lens is a +10 diopter lens. If
you stick a +2 diopter in front of the +10 diopter, the
total optical power is +12 diopter, corresponding to a
focal length of 83mm
The only problem with Canon (and other) zooms is that the
true focal length might change with the focussing distance.
For instance, the 28-135 IS USM is specified to have a
minimum focus of 0.5m and maximum magnification of 0.19x
http://www.cps.canon%2Deuropa.com/profservices/techsupp/techlib/techdoc/LTR%5F%5F0011.pdf
This can only be if the the focal length of the lens is
near 65mm (at a working distance of 0.4m)
http://members.home.net/jul.loke/
Cheers
Julian Loke
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