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Chromatic aberration, also called "colour fringing" or "purple fringing", is caused by a lens not focusing different wavelengths of light onto the precise same focal plane and/or by a lens rendering a different magnification each of different wavelengths. These two different types of chromatic aberration can both occur in one and the same image. Chromatic aberration can be seen as colour fringing around the boundaries that separate dark and bright parts of the image. It most frequently occurs around the edges of the image, especially in wide angle shots. Despite begin called "colour fringing" or "purple fringing", chromatic aberration can also affect black and white photography. Although a black and white image obviously has no colours in it, chromatic aberration can blur the image.
A special design of lens that is highly corrected to avoid chromatic aberration by bringing the three blue, red and green wavelengths to one point of focus rather than the usual blue and green of an achromatic lens. Because red is also brought to the same point of focus, you don't need to refocus when taking infrared pictures with this type of lens.