= Secondary spectrum
The <secondary spectrum> is residual <chromatic aberration> after an <achromatic lens> brings two selected <wavelengths> to the same <optical focus>. For example, equal red and blue <focal lengths> can coexist with a different green <focal length>. The <Abbe number> fixes one F–C relative difference, not the complete shape of <optical dispersion> between or beyond those <wavelengths>. The perfectly flat sum in the <achromatic doublet power balance> diagram deliberately omits this residual effect. https://imaging.nikon.com/sport-optics/guide/fieldscopes/choosing/choosing_03/[Nikon, ED glass and secondary spectrum]{ref}.
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