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An achromatic lens is a combination of concave and convex pieces of glass that focuses the different colour wavelengths in light to a single plane. The concave lens is made from flint glass and the convex lens from crown glass. Each type of glass disperses the colours differently — put together they counterbalance each other and produce a sharp image.
Right: This photograph shows the same image with and without chromatic aberration — the purple fringing around the edge of the building.
Photo: Stan Zurek.
Developed for use in telescopes, achromatic lenses are now found in all kinds of optical instruments, including cameras.
Left: Chromatic aberration and achromatic doublet based on an image by an unknown artist.