John Prosper wrote:
>Hi Folks,
>
>I've been away for a while and am now catching up on some 700
>messages.
>
>On Mon, 5 Jan 1998, Ulf Westerberg wrote:
>
>> As for lenses, a 80-200/2.8 with 72 mm thread and tripod mount and a
>> lightweight Zuiko 300/2.8 would be heaven.
>
>That 80-200/2.8 would have to have at least a 77mm filter size. The
>diameter of the entrance pupil is:
>
> 200/2.8 = 71.43
>
>Although Olympus tends to be more efficient than most other
>manufacturers at keeping their filter sizes close to their entrance
>pupil diameters of their lenses, their filter size is always at
>least 5-6mm greater.
>
True, but what about all 400/5.6's with 72 mm threads that are around?
(400/5.6 is of course also 71,43). The real answer in this real world is
that many, many lenses (especially zooms) just don't live up to their
assumed focal length.
A Sigma 400/5.6 is a good example of this, mathematically it should be more
like a 375/5.6 (375/5.6 = 67 mm). Practical tests comparing it with my Zuiko
300/4.5 and 420/6.3 (with 1.4x) confirm this completely, either the Zuiko
300 truly is a 300 mm lens or/and the 1.4x converter is more like 1.5x.
(Good for me, this difference together with a sligthly better close-range
and optical quality has greatly improved my small-bird shooting).
Zuikos are no exception to this, another example are the Zuiko zooms
35-70/3.5-4.5 and 65-200/4. Actual focal length is slightly less with the
little zoom set at 70 than the larger set at 65.
The 80-200/2.8 with 72 mm thread would then actually be an 80-188/2.8. I
doubt anyone would react to this.
Ulf Westerberg
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