For centuries the process of generating and finishing optical surfaces remained unchanged. The only suitable surface shape was a flattened sphere with an infinite radius.
Whereas it was well-understood that an aspheric surface would eliminate spherical aberration, it was simply too difficult to produce such a surface routinely.
There have been many changes since then, especially during the second half of the past century. There are now several methods available to produce not only rotationally symmetrical aspheres, but also (as previously mentioned) freeform optical elements.
For economic reasons, the spherical surface is still the most common surface. Therefore, more detailed attention will be given in our discussion to this category. This is also in line with the purpose of this text.
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