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Old December 14th, 2016, 10:12 AM
Doug Kerr Doug Kerr is offline
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Default 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:

http://www.canonrumors.com/patent-ca...le-mount-lens/

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.

Best regards,

Doug
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Old May 24th, 2017, 02:38 AM
Reginald Johnson Reginald Johnson is offline
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Doug,
I think there is a little misunderstanding here. By reversing a lens you cannot change the focal length or magnification. But by making use of the asymmetric positioning of the principal planes from the 'entrance' and 'exit' ends, it should be possible to get closer macro shots from the other end without an extender.

To change the focal length of a compound lens one has to re-position individual elements inside, such as what happens in a zoom lens.

Reginald
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Old May 24th, 2017, 11:21 AM
Doug Kerr Doug Kerr is offline
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Hi, Reginald,
Quote:
Originally Posted by Reginald Johnson View Post
Doug,
I think there is a little misunderstanding here. By reversing a lens you cannot change the focal length or magnification. But by making use of the asymmetric positioning of the principal planes from the 'entrance' and 'exit' ends, it should be possible to get closer macro shots from the other end without an extender.

To change the focal length of a compound lens one has to re-position individual elements inside, such as what happens in a zoom lens.
I certainly did not say nor mean to suggest that by reversing the lens we make any change in its focal length.

A lens does not have a magnification. The image magnification of a camera depends on the focal length of the lens and the distance from the object to the lens' first principal plane.

The "reversal" ploy is primarily intended to best exploit the lens' correction for various aberrations when used with an object at a closer distance than was considered "normal" in the design of the lens.

Best regards,

Doug
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Old May 27th, 2017, 05:41 AM
Reginald Johnson Reginald Johnson is offline
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Sorry Doug, I had misread your post.

Reginald
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