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.