The patent details a process of “adding another optical system” to another in order to reduce distortion in a fisheye lens, without increasing the size of the lens system as a whole.

without the spherical distortion. Here’s where things get really wild. I own a 15-year-old Sigma 15mm fisheye lens (available in many formats) and for this article I borrowed a Canon 8-15mm f/4L ...

barrel distortion is off the scale, but that’s what gives fisheye images their distinctive look and visual impact. It’s also one of the reasons that we don’t run lab tests on fisheye lenses ...