Properly a distortion, barrel aberration is a distortion in optical systems that magnifys the center of an image more than the edges. This results in the center of an image "bowing out." Everything will still be in focus, but points in the image will not be in their proper place.

Aberration - Astigmatism
Aberration - Chromatic
Aberration - Coma
Aberration - Curvature of Field
Aberration - Pincushion
Aberration - Spherical

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