I haven't worked out a solution yet, or maybe I have in the past and can't
remember / locate it (yet), but at:

http://news.povray.org/povray.advanced-users/thread/%3C53ff3a80@news.povray.org%3E/

clipka says,

"If you specify a camera angle, it directly specifies the horizontal
field of view; the aspect ratio (relative length of the right and up
vectors, which should ideally fit the image aspect ratio) determines the
vertical field of view."

and

"If you're not using angle, the ratio of the direction vector's length to
that of the right vector's also determines the field of view, ..."

So maybe that helps.

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