[backes]

> When we talk about "distortions" I think it's usually with regards to how the real device systematically deviates from that model.

IMO this is the correct definition of distortion. However, as the parent comment said:

> If you are not designing a rectilinear lens, there are other lens mappings, in which case it's not really proper to describe the effect as distortion, though in technical literature it's often still described this way.

I think many people confuse mapping and distortion. When a fisheye lens is used, it's often seen as "heavy distortion". But a more accurate way should be to say that it's a different mapping/projection, and the _distortion_ a calibration measures is the difference of this ideal projection (ftheta (e.g. Kannala Brandt) rather than fthan*theta (pinhole) ), and the actual image. This can be a minuscule amount. This means that, "undistorting" a fisheye image doesn't give you a rectilinear image, but still a fisheye image. You can of course decide to map the undistorted fisheye image to a rectilinear one, but that's conceptually a different operation than (un)distortion.