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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1557934" data-attributes="member: 68323">So that's the same number (given it has an internal focusing lens) as a porro (assuming the EP and objective are the same). The AK roof would be two less glass/air transitions whencomparing like with like i.e. for a porro with an internal focus. And the AK prisms use total internal reflection for all their reflections just like porro prisms.For Schmidt-Pechan roof prisms there are two extra glass/air transitions. Plus one reflection in the prism has to come from a reflective surface not TIR.All roof prisms need phase compensating coatings to keep the resolution the same for all directions through the prism.That said though with modern AR, dielectric mirror and phase coatings there is little effective difference between the two in terms of transmission but roof prism components are more expensive to make because they have to be made to higher tolerances or require additional coatings.</blockquote>