A lens may be imperfect and may distort or alter light. In order to prevent these problems modern lenses have become complex devices. Identification of a particular problem can help identify the best solution to use.
Defects Affecting the Entire Visual Field
Defect
Features
chromatic aberration, axial
light of different wavelengths will focus at different points, with an image appearing to be surrounded by a fringe that varies in color depending on the focus
spherical aberration
the point of focus for axial, medial and peripheral rays are different, with the an image not sharply focused but rather showing a "zone of confusion"
Defects Affecting Points Off Axis
Defect
Features
chromatic difference in magnification (chromatic aberration, lateral)
unequal magnification of light of different wavelengths affecting peripheral rays (for example, a red object may appear larger than a blue object)
coma
points off-axis are focused in a conical fashion resulting in a blurring of the image
astigmatism
unequal curvature with the radius of curvature in one plane different (either longer or shorter) than the radius in the plane at 90° to it
field curvature
the field of focus is curved rather than flat, with the edges curved towards the lens
distortion, barrel type
the sides of a rectangular structure curve outward from the center
distortion, pincushion type
the sides of a rectangular structure curve inward towards the center
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