Had a customer ask, the other day, why most astronomy binoculars are porro prisms, rather than roof prisms.
The answer is really a matter of efficiency.
Roof prism binoculars are not as inherently efficient for light transmission as
porro prism binoculars, since roof prisms incorporate a mirror surface and, even with expensive, high reflectivity mirror coatings, there is some light loss at the mirror surface. Roof prisms also need to be phase coated to produce the sharpest possible image. Porro prisms, on the other hand, don't use a mirror surface or need phase coatings and are thus typically brighter than a comparable quality roof prism. In addition, porros are less expensive to produce to high optical standards than a roof and the main advantages of a roof prism, namely sleek handling and internal focusing, are not really issues for astronomy. Even the finest porro prism binocular, such as a
Nikon 12x50 Premier SE (one of the finest optics made, period) can be bought at a fraction of the price of a premium grade roof such as the
Leica 12x50 Ultravid.
For more on the basics on astronomy binoculars, see our article,
Astronomical Binoculars