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For a camera with a rectilinear lens a simple measurement would consist of taking images from known distance of an object of known size. For example using a sheet of paper as an object set on a wall perpendicular to optical axis of the camera and taking images from several distances to have an average value (or do linear fit). Then say horizontal fov can be calculated from images as
hfov = 2 * arctan(Img_w/(2*d))
Img_w = Img_wp * (Obj_w/Obj_wp)
Img_w - image width [mm]
Img_wp - Image width in pixels
Obj_w - object width [mm]
Obj_wp - object width in pixels
d - distance to object [mm]
As I understand, for rectilinear lenses and fixed distance to the object the ratio Obj_w/Obj_wp would not change regardless of where the object is located in the image. But for lenses with distortion like fisheye, this ratio would depend on the object location on the image and the method would not work.
Is there a simple similar method to measure fov for fisheye lenses ?
What method is used in the industry to measure fov of camera systems ?
Since I have a camera a with small S-Mount lenses and working distances are about ~1 meter, one possible way could be to have a ruler or tape measure set on a wall spanning the whole image horizontally. Then calculating the fov from the distance d and scene width seen on the tape measure in the image. But I'm not sure if that would work.