One additional thought on this:
In a rectilinear lens (maps a flat field in critical focus to a flat film
plane), dead-center on the lens axis is the closest point to the lens of
the flat field in critical focus.  [As I understand it, the focus markings
are for the axis, not the corners.]  As you move off-axis the distance
grows.  The corners are farthest away.  The wider the AOV, the greater the
corner distance compared to the axis distance.  With a super-wide the
effect can be so great, that someone's head in a corner can have a lower
corner of the chin enough closer than an upper corner of a forehead that it
looks like a huge chin with an off-center pointed head.
-- John
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