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Message: <blockquote data-quote="Leif" data-source="post: 1306483" data-attributes="member: 1250">I have no idea if the roof prism does or does not introduce any noticeable image degradation when well made and coated. But I recall someone stating that roof prism bins tend to have faster objectives - i.e. shorter focal lengths - which will introduce aberrations. In addition faster objectives will require more sophisticated eyepieces, which will increase cost, and perhaps extra aberrations from the eyepieces. In addition most roof prism bins focus by means of a sliding lens between the objective and the prism. My suspicion is that the focus lens is a major cause of CA in many roof prism bins, especially cheap ones. To my eye roof prism bins which focus by moving the objective tend to have noticeably better image quality than others. Anyway, I doubt that major manufacturers will introduce top grade porro bins, sadly. Fashion, weight and bulk surely mitigate against them.</blockquote>