An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better color correction than the much more common achromat lenses. Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens. In photography, it produces soft overall images, and color fringing at high-contrast edges, like an edge between black and white. Astronomers face similar problems, particularly with telescopes that use lenses rather than mirrors. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. Apochromatic lenses are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (secondary spectrum) can be up to an order of magnitude less than for an achromatic lens of equivalent aperture and focal length. Apochromats are also corrected for spherical aberration at two wavelengths, rather than one as in an achromat.