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Canon patent for a reversible lens
The design of familiar "general purpose" lenses, unsurprisingly, usually arranges for the various compromises involved in correcting for aberrations to "work out best" when the lens is used within its intended (and, for a system lens, normally available) range of image magnification.
Sometimes we (perhaps, for a system lens, with the use of an extension tube) are able to arrange for the lens to provide greater magnification than usual, as may be desirable for the photography of smaller objects.
But when we do so ("off-label" use, as they would say in the pharmaceutical business), we may no longer enjoy the correction of aberrations intended by the lens designer.
A well known ploy in this situation (especially for operation at magnifications substantially greater than one) is to arrange for the lens to be mounted "reversed". Then the lens "thinks" it is operating at a lesser magnification (it doesn't, after all, know which side of it is the object space and which the image space). We then enjoy, essentially, the aberration correction the lens would exhibit when operated normally at that lesser magnification.
A recent Canon patent, mentioned on Canon Rumors here:
seems to permit the convenient mounting of a lens either way to, in connection with that "ploy".
It will be fascinating to see what emerges from this yet-unhatched egg.
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