Porro or Roof? That is the question!
2. Roofs more complicated
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There are more, worth to mention, things about roof prisms which are of great importance in image quality. First of all there are distinguished two types of roof prism binoculars. Abbe-Koenig prisms which similarly to porro take the advantage of phenomena of total internal reflection so light loss is really insignificant. The second system - Schmidt-Pechan prisms use reflection from a mirrored surface. Quality of this surface translates into quality of received image. Also quality of this surface bear responsibility for light transmission. Typical aluminum coating contributes to even 10% light loss whereas silver mirror coating or dielectric coating light loss is only on 0.5-1.0% level but this coatings are definitely more expensive and used only by reputable manufacturers.
Constructors of binos also grapple with previously mentioned phase differences of light beam which first are split and the rejoined. Even slight phase shifts cause that leaving a prism light waves, in total beam are partially suppressed. As a result of this phase differences, the final image has a lower contrast and worse quality. In addition the higher magnification is applied, the worse quality is generated. Reputable manufacturers, to minimize the phase difference effect, use a special so called phase coatings. That’s another example why roof binoculars are more difficult to design.
