I have a 180° equisolid (equal-area) fisheye lens with a focal length of 8 mm. I want to crop images to a circle defined by an angular field of view from the zenith angle of the lens (i.e. the middle of the circle in the original image).
I have already found an equation which I think may be on the right lines:
R = 2ƒ sin(θ / 2)
Where θ is the angle (in radians) from zenith defining the circle to crop to (i.e. my given field of view), and ƒ is the focal length of the lens (i.e., 8 mm). R is the radius of the circle drawn by the field of view (I think this is the radius on the sensor plane?), in mm. The issue is I can't figure out how to relate R to a pixel length on my images so I can crop to a circle of that diameter.
I've already looked at these pages: