The Sigma 400 mm f/ 5.6 APO is a telephoto lens for 35-мм SLR cameras of major autofocus systems.
APOstands for Apochromatic lens.
Sigma apochromatic lenses reduce chromatic aberration to a minimum. Since refraction index of light depends on wavelength of light, chromatic aberration occurs when the lens fails to focus all colors to the same convergence point. This problem is especially characteristic of telephoto lenses, but low dispersion glass elements (SLD, Special Low Dispersion, and ELD, Extra Low Dispersion), applied in Sigma apochromatic lenses, effectively prevent chromatic aberration and improve pictures quality.