Purple fringing is a rather poorly-defined term used to describe out-of-focus purple or magenta fringes seen most often around back-lit and high-contrast subjects. The effect is sometimes described as a "ghost image". Lenses are designed to reduce chromatic aberration by focusing wavelengths (colours) in the visible spectrum, or at least three of the principal wavelengths, in the plane of focus and with minimal lateral separation. However, wavelengths outside the visible spectrum, such as ultraviolet and infrared, may remain significantly out of focus.
Although film has a low sensitivity to such wavelengths, digital sensors retain sensitivity over a much wider range of wavelengths. Various measures are adopted to reduce or eliminate ultraviolet and infrared components, but sufficient energy may remain to cause visible effects.
Microlenses on the surface of sensors in digital cameras also contribute to the phenomenon of purple fringing. Their effect may be considered as chromatic aberration at microlens level. Unlike true chromatic aberration caused by a camera's lens, the microlens phenomenon is visible throughout the image. Blooming tends to increase the visibility of purple fringing.