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02-11-2010, 05:36 AM   #1
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Reversed lens image circle?

I ask this in anticipation of using a reversed lens in a tilt-shift application. I'll take a suitable lens longer than about 60mm and reverse it to move it farther from the sensor.

For example, the front focal point for a 100mm lens lies somewhere around 100mm from the front of the lens. This is more than enough distance to accommodate tilting the lens with respect to the optic axis. but is the image circle at that focal distance the same as it would be if the lens were mounted normally?

Would a full frame lens have the same sized image circle when used in reverse? Like I reverse a 100mm lens and mount it such that the focal point for the reversed lens falls on the sensor; the entrance and exit pupils have been swapped. Does the image circle stay the same or does it change according to some rule?


Dave in Iowa

02-11-2010, 07:14 AM   #2
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The relative sizes of the coverage angle on either side of the lens depends on the type of lens. On a conventional semi-symmetric prime focus lens (tessar, biotar, planar etc) the angular lens coverage should be the same on either side of the lens. On a retrofocus type wide angle lens the coverage angle at the back (what is normally the film side) will be smaller than that at the front. For a telephoto (reversed retrofocus) lens it should be the other way round.

Reversing a lens and using it at infinite conjugates (objects a long way off) may not work that well as the lens has been optimised to work the other way round. This will degrade the performance but I don't know how badly (will depend on the lens).

Your assumption that the front focal point will be the focal length away from the front of the lens may bot be correct. The front focal point will be the focal length away from the front principal plane of the lens and the back focal point will be the focal length away from the back principal plane. Both principla planes will usually be somewhere inside the lens and separated by a distance depandant on the lens. With standard lenses (most semi symmetric 50mm designes) both principal points tend to be closer to the back of the lens. This would mean that reversing the lens would reduce the distance you have rather than increase it and make it impossible to focus to infinity. This is why remerse mounting a lens makes it work as a macro lens, it has the effect of adding extension tubes.
02-11-2010, 08:16 AM   #3
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My experience has been that telephoto designs don't reverse very well.
You are best sticking to shorter lenses.
02-11-2010, 08:37 AM   #4
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The main problem in that position is, that you can only focus very near. Probably not even enough for table top applications.

Your best bet for a shift-tilt application is a medium or large format lens.

Ben

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