Double lens: adding lens powers<br>L<sub>1</sub> = lens 1, F<sub>1</sub> = focus lens 1, <i>f</i><sub>1</sub> = focal length lens 1,<br>L<sub>2</sub> = lens 2, F<sub>2</sub> = focus lens 2, <i>f</i><sub>2</sub> = focal length lens 2
<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>