I’d use the SMIA TV distortion operand as it doesn’t require a paraxial reference, and it’s what is often quoted in lens specifications. Failing that, just REAY/X operands to target a specific field to a specific image location.

Distortion control in a Fisheye lens is hard, and it’s not because of a lack of operands. The fisheye takes an large angular range and outputs a smaller angular range for the prime imaging system to image. It’s inherently anamorphic, like the Mapmaker’s Problem. Most people don’t try to control distortion in a wide angle lens, and it’s often not even specified (it might be quoted on the spec sheet, but it wasn’t used as a design parameter). Remember that the entrance pupil of a wide angle lens is not constant, it moves and rotates as a function of field. That’s what makes the lens ‘wide angle’.

• Mark