Lens Correction - Pentax K-x Operating Manual

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You can also change the setting from the [A Rec. Mode 2] menu (p.79).

Lens Correction

Reduces distortions and lateral chromatic aberrations occurring due to
lens properties.
Distortion
Distortion is the phenomenon in which the center of the image appears
inflated (barrel distortion) or the center of the image appears pinched
(pincushion distortion). Distortion occurs more easily when using a
zoom lens or a lens with a small aperture, and straight walls or the
horizon in the image appear curved.
6
Lateral chromatic aberration
Lateral chromatic aberration is the phenomenon in which the
magnification of the image varies according to the colors (wavelengths
of light) when a picture was taken, and may cause a blurred image.
Chromatic aberration occurs more easily at shorter focal lengths.
Pincushion distortion Barrel distortion