An achromat corrects chromatic aberration at two wavelenghts (not to scale, effects exaggerated)<br>F<sub>r,b</sub> = focal point for red and blue, F<sub>g</sub> = focal point for green
<a href="//commons.wikimedia.org/wiki/User:HHahn" title="User:HHahn">HHahn</a>
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<span class="int-own-work" lang="en">Own work</span>