Light that goes through the center of a lens can be focused to a point. If the light goes through the lens off axis (at an angle) the light will not focus to a point and look like a fuzzy circle. The farther off axis, the larger the circle, giving objects a comet-like look to the image (hence the name "coma").

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

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