Porro Prism vs. 45degree erect Image

Could someone explain the difference between a "porro prism" and a 45 degree erect image diagonal. Also the pro's and con's of either.

Regards,
Jared

Porro prisms are like what is in most binoculars, so they have a straight-through view given by 4 internal reflections. The 45 degree versions use a single prism and, as the name implies, angle the light path by 45 degrees. Since both cause the light to go through lots of glass, both are primarily for daytime use. My guess is that a cheap 45 might perform better than a cheap porro (since it is only one piece of glass). Right now I am waiting on a 45 with a large clear apperture for daytime use with a 32mm Plossl (or 24 Pan when I can aford it). Most of the ones you get will limit the field of view somewhat (also true of the one porro I have used). I guess your preference would depend on how you want to hold your head when viewing. Hope this helps.

Charlie