Apochromatic (ED APO) lenses are designed to bring all the colors into focus in the same plane where as Achromatic lenses are corrected to bring two wavelengths into focus in the same plane.
Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens. Telescope objective lenses for wide-band digital imaging in astronomy must have apochromatic correction, as the optical sensitivity of typical CCD imaging arrays can extend from the ultraviolet through the visible spectrum and into the near infrared wavelength range. Apochromatic designs require optical glasses with special dispersive properties to achieve three color crossings. Explore Scientific achieves this using high-quality ED (Extra-Low Dispersion) glass from Hoya and Ohara.