What is the difference between Porro prism and roof prism binoculars?Updated 2 days ago

Binoculars with Porro prisms can be identified by a noticeable offset between the objective lens and the eyepiece, with the objectives typically positioned outward in the binocular arrangement of the two telescopes. This design is based on the configuration patented by Ernst Abbe in 1893, which laid the foundation for binocular construction. Porro prisms come in two different configurations, with the first type of Porro system being the most prevalent today.
From around the 1920s, binoculars with other prism reversal systems were developed and produced in larger quantities. These systems are characterized by a reflective surface shaped like a roof edge. They can be constructed without any offset between the incoming and outgoing optical axes. Typical roof prism systems include the Abbe-König, Uppendahl, and Schmidt-Pechan designs, the latter being the most widely used today. Optical performance:
Porro prism systems avoid the inherent disadvantage of roof-shaped prism sets, namely polarization effects. These effects in roof prism systems can only be minimized through extremely high precision and the application of phase-correcting coatings on the roof surface.
In a Porro prism system, the reflection on the cathetus sides of the prisms utilizes total internal reflection making it almost loss-free. This eliminates the need for sensitive, transmission-reducing mirror coatings.
By cementing the two individual prisms together, as employed in systems like those from NOBLEX, the number of optically active surfaces is reduced from 8 to 6. This reduces potential negative effects on image quality while simultaneously increasing light transmission by eliminating two glass-air surfaces. In contrast, a Schmidt-Pechan roof prism system has 10 active surfaces that influence the image.
The clear beam passing through the prism system, and the prevention of stray light through appropriate prism mount designs, enable a high-contrast image. The occurrence of bright stray light exiting through the eyepieces (secondary pupils), which even high-quality roof prism systems cannot always entirely avoid, is completely eliminated and ensures high image brilliance.
A Porro prism system can handle significantly larger bundles of light, guiding them through the prisms without loss. Typically, for the same objective diameter, the light bundles in Porro prism systems are twice as large as those in roof prism systems and are fully transmitted. This results in a brighter image even toward the outer edges, making Porro prism binoculars better suited for twilight conditions as compared to roof prism designs.

(Source: A. Köhler from www.akoehler.de)