Can a split diopter bring two different optical path lengths into focus in one shot?

Question

I’m doing fixed-position high-speed scientific imaging with a Micro Nikkor 105mm lens. The camera sees a subject directly from the side, and also from below via a small mirror at 45° under the subject. Because the mirror view has a different optical path length, I can’t get both the side view and underside view sharp at the same time.

Would placing a small convex lens in only one part of the image path—so that one view passes through it before entering the camera lens, while the other view does not—allow both views to be in focus at once? If so, what kind of lens/filter is this, and what math is typically used to estimate the needed optical power?

user11924 · Answer

Cinematography solved this problem long ago with the split diopter. It's literally just half of a screw-on diopter, just like so-called "close up filter" (although it doesn't filter anything, it's called that because it mounts to the lens like other front-mounted actual filters).

Split diopters don't see much use in photography, but they used to be used in film a lot, when the director wanted to draw the viewer's focus simultaneously to a near subject and a far subject. They're still used in film, but with today's CGI and compositing, the effect is now usually done in software with multiple takes, rather than in-camera in the same shot.

This article talks about several classic cinema shots achieved with split diopters.

^{split-diopter shot from All The President's Men}

^{split-diopter shot from Jaws}

Can a split diopter bring two different optical path lengths into focus in one shot?

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