When a lens is corrected for three colors, it is described as what?

• A. Apochromatic
• B. Achromatic
• C. Spherical
• D. Plan

Answer: (A)

Apochromatic describes a lens designed to bring three wavelengths of light to the same focal point, greatly reducing chromatic aberration across those colors. Chromatic aberration happens because glass bends different colors by different amounts, causing color fringes. An achromat corrects two wavelengths (typically red and blue) to align their focus, but leaves residual color at other wavelengths. An apochromat uses special glasses and extra elements to align three wavelengths (often red, green, and blue) and usually provides better control of chromatic and spherical aberrations. Terms like spherical relate to a different type of distortion from lens shape, and plan refers to a flat, non-curved surface—not color correction. So, when corrected for three colors, the lens is apochromatic.