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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1525419" data-attributes="member: 68323">It's not alignment it's in the making of the roof prism.The roof on the roof prism needs two things to work well:1. the roof angle has to be 90 degrees with very low error (seconds of arc unlike the minutes of arc required for a porro prism) otherwise you see a gap or overlap between the two "halves" of the image.2. the roof edge has to be straight and sharp (so that a minimal amount of light is diffracted off it and that that is is diffracted over a wide angle by a sharp edge (less than a wavelength of light)These both test the ability to construct roof prisms. And this is one of the reasons Leica and Zeiss stopped designing porros in the 1960s to focus on roof prisms: it takes more technical effort to get right.But it can be done ... of the three Chinese EDs I have only 1 barrel shows any diffraction (and it's smaller than the Made in Japan Eaglet).</blockquote>