At 14:12 03.10.02, Julian wrote:
The theory of the reversing ring is to exploit exactly those corrections
for distant focusing when the relationship between object distance and
flange - back becomes abnormal (ie the distance from the lens to the film
becomes greater than the distance from the subject to the film. The
assumption is flawed in that the distance from lens to film will never get
into the optimum working range for the lens, unless the lens is already
capable of high quality close focus, at which point you just use it non -
reversed anyway etc etc. So for a lot of hassle, you get a minor improvement.
Extension rings are not a better answer as you just push a lens further
outside its optimum working range.
This all sounds very good untill you think about the conditions under which
it applies. At 1:1 the lens is just as good/bad either way it is pointed, I
hope we can agree on that. To reach this point you'll already need about
50mm of additional extension for a 50mm lens. Turning it around gives a few
extra mm extension from the ring itself and the lens body (because the lens
element are skewed to the back of the lens body), but nowhere near 50mm, so
you'll still need extension tubes to get beyond 1:1, where the marginal
improvement is.
You are right that more extension just pushes the lens further out of it's
optimum range. But it is not untill you cross the ~1:1 barrier that there
is improvement to be had from reversal, i.e. the lens is pushed back
towards optimum range. The lens is optimized for infinity (1:infinity), so
the *optimum* macro range would in fact be infinity:1. That would of course
take one **** of a bellows, take forever to expose, and have zero field of
view ;-)
As I've already said, reversing rings can be useful with wide angle lenses,
but only to gain high mag with short bellows draw. You get all the other
(expensive) disadvantages from high mag either way you go about it, so if
the cost of a real macro lens seems too high....
Regards,
Thomas Bryhn
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