What does a lens distortion rating like 4% barrel distortion mean in a photo?

A 4% barrel distortion rating describes how far the image bows outward from an ideal rectilinear rendering. In simple terms, straight lines near the edges of the frame will curve outward slightly instead of staying perfectly straight.

The percentage is typically expressed relative to the frame height. If you photographed a perfectly rectangular subject that should fit the frame exactly, barrel distortion would make the image “underspill” at the corners—the subject’s corners would fall slightly inward compared with an undistorted image. Pincushion distortion does the opposite and tends to “overspill.”

So 4% means the deviation at the edge is about 4% of the image height. In practice, that is generally a small amount and may be only mildly noticeable, depending on the subject. It tends to matter most when photographing architecture, horizons, or other subjects with straight lines near the frame edges.