>
> Charles wrote:
>
> >There is no glass so it is very light to carry and there is no image
> >degradation.
>
> Years ago there was a test of macro lenses, and in one sentence they said,
> that the extension tubes don't degrade the image quality, because there are
> no lenses inside. The funny thing was, that the Tokina 90mm 1:2,5 was the
> lens with the best 1:1 performance with its extension tubes with three
> elements. In this test, there were some general "good" original
> manufacturers macro lenses with an "insufficient" performance at 1:1, some
> of them used extension tubes without optical elements. So their conclusion
> and their own test results conflicted.
>
> Matthias
Extension tubes do degrade the image -- by causing the lens to operate at
a focal distance it may not have been optimized for. Many lenses, for instance,
develop substantial field curvature when extended. A converter optimized for
a lens may give superior performance.
Paul Farrar
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