Porro prism binoculars have objective lenses that are set wider than the eyepieces. Images seen with Porro prism binoculars are brighter than images with the roof prism type, which is due to the differences in their internal optics, the placement and type of prisms.

Actually, there is now a third type of binoculars and they are called reverse Porro prism binoculars. These binoculars are less bulky than the original Porro prism type. The eyepieces are set wider than the objective lenses, but you still get the benefit of a brighter image from the Porro prism arrangement.

Company Resource:      Big Porro Binoculars, Compact Porro Binoculars
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