For optical systems used in VR, several lenses will be combined in succession. What is the effect of the combination? A convenient method to answer this question with simple arithmetic was invented by ophthalmologists. The idea is to define a *diopter*, which is . Thus, it is the reciprocal of the focal length. If a lens focuses parallel rays at a distance of m in behind the lens, then . A larger diopter means greater converging power. Likewise, a concave lens yields , with a lower number implying greater divergence. To combine several nearby lenses in succession, we simply add their diopters to determine their equivalent power as a single, simple lens. Figure 4.16 shows a simple example.

Steven M LaValle 2019-03-14