"Reflection is an interesting word with many meanings. To a philosopher it means careful or considerable thought on a subject. To an anatomist it means a structure that bends back along the path it came from. To a narcissist it’s their image in a mirror. To a photographer it usually means subjects mirrored in water, ice, or glass.
But to a lens designer (or just a gearhead) it refers to light that bounces off of the glass elements in a lens rather than refracting properly to form an image on the sensor. It’s this meaning that I’m reflecting on today. Most people don’t think about how huge a problem this is in lens design and what enormous effects it can have."