Given an ellipse with semi-major and semi-minor axes
and
, we look for viewpoints from which the ellipse subtends the angle
. We start from the point
on the ellipse; this will be one of the tangent points. Next we calculate the two possible viewpoints
,
; from one of them the ellipse subtends the angle
, from the other one the ellipse subtends the angle
. As the parameter
goes from 0 to
we get the set of viewpoints for those two angles, which form the two iso-optic curves of the ellipse.
You can set the ellipse parameters
and
to between 1 and 10, and
between 5° and 90°. The first snapshot shows the iso-optic curve in general. From the second snapshot you can see that if
is small, the curve looks like a circle. In the third snapshot,
, so the curve is a circle.
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