Chromatic Aberration

A separation of the different colors of white light resulting in color fringing or color halos in a photo. This commonly occurs when an under-lit subject has an intense backlight. The result can be a purple “fringe” surrounding the subject.

Chromatic abberation is more common in consumer level lenses, but can be seen in even pricier models.

A simple test: shoot at sunlight that is filtering through a tree. If a lens does not fully compensate for chromatic aberration, purple fringing typically will be visible around the sihouettes of the leaves as the sun’s rays pass around them.

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Illustration: chromatic aberration
Illustration: achromatic lens - a lens that corrects for chromatic aberration.