"You're not afraid of the dark, are you?" - Riddick "The best scientists are humble. They seek to understand, not to ensure their legacy, but merely to understand." - Mori
Quote: When dealing with the optical thickness of prisms, it must be borne in mind that the glass moves the focus rearward by an amount roughly equal to 1/3 (depending on the index of refraction) the optical path length through glass. In effect, then, a prism is optically thinner than it is physically, the equivalent air path being about 0.67 that of the glass oath length. We can see that in this application prisms offer the benefit of squeezing some extra back focus into the train.