Porro prism

An image travelling through a Porro prism is rotated by 180° and exits in the opposite direction offset from its entry point.

Such an abaxial ray then emerges from the prism having been reflected a third time, thus introducing non image-forming stray light and reducing contrast.

The net effect of the prism system is a beam parallel to but displaced from its original direction, with the image rotated 180°.

Sometimes, the two components of the double Porro system are cemented together, and the prisms may be truncated to save weight and size and reduce glass/air transition surfaces to two and hence light transmission loss.

Porro prism of the second type optical systems are not very common and generally applied in larger and military binoculars.

[8][9] Traditionally binoculars used a double Porro prism design, which resulted in a distinctive offset, zig-zag shape.

As human eyes are ergonomically limited by their interpupillary distance the offset and separation of big (60+ mm wide) diameter objective lenses and the eyepieces becomes a practical advantage in a stereoscopic optical product.