At 02:06 2/2/02, Moose wrote:
The Vivitars are well built and work fine. They are different lengths than
the Olys. Viv= 12, 20 & 36mm, Oly = 7, 14 & 25mm. Someone, perhaps John
Lind, recently propounded on the reasons for the differences. I use An Oly
7mm plus the Vivitar set.
The OM tube lengths were selected to allow continuous magnification using
combinations of the tubes plus the focusing ring on the 50mm Zuiko lenses.
I have "deduced" from this the Vivitar tubes are designed for a longer
lens; likely about 85-90mm.
My Logic:
The focus ring on a prime will extend the lens about 1/7th its focal length
farther from the film plane.
7 x 12mm = 84mm; 12mm being the shortest tube in the Vivitar AT-21 tube set.
Why "non-macro" primes do not extend much beyond 1/7th focal length:
Nearly all primes are focused to a distance closer than infinity by
extending the entire lens cell farther from the film plane. As a result,
the lens' image circle gets bigger as you focus closer. This spreads the
same light over a larger area. Macro lenses are designed to compensate for
this; non-macro lenses do not. The limit before overall falloff starts to
approach 1/4 to 1/3 stop is about 1/7th the lens' focal length.
-- John
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