Peter J. Alling mused: > > Brighter with easier focusing and viewing. I don't know if that's > inherent or that it's just easier to make > a pentaprism that way.
Inherent. The reflecting surfaces in a pentaprism rely on total internal reflection, which is to all intents and purposes 100% efficient. There's going to be some light loss at a mirror, so by the time the image has bounced around inside a pentamirror there's going to be measurable light fall-off.
