Lenses bring parallel rays of light (alternatively, light from infinitely far away) to the focal point. They don’t bring idealized points to points. One conseq

Lenses bring parallel rays of light (alternatively, light from infinitely far away) to the focal point. They don’t bring idealized points to points. One consequence is you can’t use lenses to bring anything to a temperature higher than the temperature of the source light. For example you can’t use lenses + moonlight to light things on fire. Here’s an HN thread going into the physics of it: https://news.ycombinator.com/item?id=18736700 https://news.ycombinator.com/item?id=18736700