Inverted, Reverse or M shapped Porro Prism Binoculars

In binoculars for a more compact design, the objective lenses can be placed closer together,
resulting in the 'M' or inverted porro prism style as illustrated.
The closer spacing of the objective lenses in this configuration tends to reduce the stereoscopic
effect when compared to the standard porro prism configuration

The roof prism binoculars are more compact than the porro prism.
On the other hand in Porro prism the reverse, inverted or "M" shaped compact binoculars are small, nice lightweight designs. The difference is ilustrated.
Nikon, Canon, Pentax and other big brands use this sort of prism