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Message: <blockquote data-quote="henry link" data-source="post: 3432704" data-attributes="member: 6806">I think Pier is referring to vignetting at the field edge, which according to the patent ( <a href="http://www.google.com/patents/US20120140349" target="_blank">http://www.google.com/patents/US20120140349</a> ) is a little worse in the Perger Prism than in a standard Porro Type 2 (see Summary [0005] and Detailed Description [0019]). However, light transmittance over most of the field would be exactly the same in a cemented Perger as in a cemented conventional Porro 1 or 2, or a cemented Abbe-Konig. All of those would be a little higher than an air-spaced Porro or A-K and higher still compared to an air-spaced Schmidt-Pechan. We can't tell from the patent just how wide a field could be accommodated before the extra vignetting of the Perger becomes obtrusive. It would vary depending on the rest of the design.Henry</blockquote>