I know what barrel distortion is, but how do I take a 4% rating and relate that to an image?
We desire a faithful image. We image a rectangle with the same aspect ratio as our camera’s senor/film frame. If we image at unity (life-size) we expect the image of the rectangle to kiss-off at the corners of the image area. With barrel distortion the image will under spill. With pincushion distortion, the image will over spill. The delta (valuable increment) is expressed as a percentage of the height of the frame. 4% is scarcely noticeable.