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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1306555" data-attributes="member: 68323">It works exactly at one frequency and less well at small offsets (the error increasing away) when you build a stack you choose a series of frequencies and optimize each of these.This is exactly the way you design AR, dielectric mirrors and other thin layer systems.You make the mistake of "infinite number of frequencies". There is but you can't perceive them all. You have three different detectors (cones) in the eye. Each works over a band of frequencies and your brain devotes a lot of effort in assigning colors to portions of the image. You eye isn't perfect. It isn't a spectrometer. So you have to make the correction only good enough to work within the limits of the detector which in this case is three color bands. In real lilfe you optimize across an octave or so (the wide of visible light band) but you bias the curve to the eye's response (which isn't flat either).<a href="http://en.wikipedia.org/wiki/Color_vision" target="_blank">http://en.wikipedia.org/wiki/Color_vision</a>It's all about tolerances. There are no perfect systems out there. Including the eye.BTW, with your logic you can't use any AR in your ideal porro because they're optimized for one frequency (single layer AR) or only a few of the many frequencies.Read a bit about approximation theory if you don't get any of this.<a href="http://en.wikipedia.org/wiki/Approximation_theory" target="_blank">http://en.wikipedia.org/wiki/Approximation_theory</a>I'm sure the M24 is bought under duress too  <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />. For the M24 the design goal was to make a "compact" bin that would fit into a BDU pocket usable by any solider. This constraint wouldn't be met by a 7x30 porro. It doesn't fit.But I suspect the general "porro versus roof" for the military comes down to cost. The porros are cheaper than roofs and for most military uses compactness or even light weight is not the issue with plenty of fit twenty something users. You can build porros today as robust as roof (pretty much - the eyepieces are a bit more vulnerable to a shear force because they're not in a single tube).I didn't say that. I think you misread something I wrote. The light output from porros though is polarized (due to reflection) but that doesn't have any effect on resolution. Maybe you misread that?100x worse, eh?You know that 97.2% of statistics are made up? As I've said multiple times now: manufacturers make more money from roofs.People buy them because they like them. Quite a few people with good eyes for optics. That's why they make the roofs. They don't make the porros because they don't make as much money and (as you say) they would be just as good (not a lot better). The good roofs actually work fine (despite your protestations) and make the makers a lot of money. QED.The only way out of this is a new company. To sell to logical cheapskate buyers (i.e. the military) or to make porros so attractive (i.e. cheap and good) that people flock to them despite them being bigger (and for quite a few more awkward to handle). But to do that you need a company that doesn't make roofs. It's a market gap but I suspect people like their ergonomics too.I think it's quite clear that this is a religious argument not a logical one.</blockquote>

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