色差是指光学上透镜无法将各种波长的色光都聚焦在同一点上的现象。它的产生是因为透镜对不同波长的色光有不同的折射率（色散现象）。对于波长较长的色光，透镜的折射率较低。在成像上，色差表现为高光区与低光区交界上呈现出带有颜色的“边缘”，这是由于透镜的焦距与折射率有关，从而光谱上的每一种颜色无法聚焦在光轴上的同一点。色差可以是纵向的，由于不同波长的色光的焦距各不相同，从而它们各自聚焦在距离透镜远近不同的点上；色差也可以是横向或平行排列的，由于透镜的放大倍数也与折射率有关，此时它们会各自聚焦在焦平面上不同的位置。

In optics, chromatic aberration (CA, also called achromatism, chromatic distortion, and spherochromatism) is an effect resulting from dispersion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have different refractive indices for different wavelengths of light. The refractive index of transparent materials decreases with increasing wavelength in degrees unique to each.