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โOptics and Symptoms
The horizontal โsqueezeโ of an anamorphic lens is produced by having two different focal lengths, one in the horizontal direction, and one in the vertical direction. That effect is produced by a stack of (moving) lenses that have different curvatures horizontally and vertically, unlike normal โsphericalโ lenses that are perfectly symmetric about their central axis.
Depending on the design of the lens, the horizontal and vertical axes may have different entrance pupil locations โthe center of the entrance pupil is the โnodal point,โ where the camera is located for matchmoving purposes. The distance between the two is the anamorphic distance (which can be positive or negative).
If there are two different nodal points, then the distance from any object to the nodal point(s) is different for each axis!
Does having different horizontal and vertical focus sharpness in anamorphic lenses sound familiar? Thatโs what youโd expect with two different distances to the same thing, right? One is in focus, the other, not so much.
How about aspect ratio changes during focus breathing? As the component lenses shift around inside the overall anamorphic lens to change focus, those nodal points can shift differently from one another, so the relative proportions and aspect change.
That brings us to the infamous โanamorphic mumps ,โ where actors could have fat faces up close, yet be skinny far away. Anamorphic mumps were solved long ago, right? Not so fast. Hereโs a great quote from cinematographer David Mullen:
"Fat faces" from shooting anamorphic, if that's what you meant, is an ancient problem called "anamorphic mumps" as was solved in the early Panavision lenses of the late 1950's. It was only a problem with early CinemaScope, where as you focused closer and closer, the compression ratio changed, not squeezing the in-focus object enough (less than 2X) so that when uncompressed by twice in the theatres during scope projection, the object looked fat. Panavision solved this by making the compression error occur in what's OUT of focus as you focus closer and closer, so the out of focus background gets compressed MORE than 2X, making it look skinny even after itโs expanded by twice during projection.
That underlined sentence should get the immediate attention of all matchmovers! Panavisionโs fix works only on the subject in focus, ie typically on the actors. It did that by moving the problem into the backgroundโwhich is exactly what we must track as matchmovers.
Mumps arenโt fixed, they are simply hiding.
For a few more words on the two different nodal points from a cinematography standpoint, thereโs โ Pitfalls of Anamorphicโ by Art Adams; it is also discussed in the lens design and camera calibration communities.
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