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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1727991" data-attributes="member: 68323">The manufacturer is making a set of trade offs (as with all optics) in all bins. So that answer is "Yes". It depends on the market.IF focusers are just easier to seal than CF focusing bins: the bin is simpler and more rigged. That's why they get used. They're also more convenient in non-birding applications where the target is away at or closr to "infinity" (i.e. more than 50 to 100m away for most accommodation). And where you don't want the bins accidently knocked out of focus e.g. military or ship board uses. Of course they're useless for following a warbler (you'll you are really dexterous!).A smaller FOV enables simpler EPs to be used reducing cost (and manufacturing time).A smaller FOV reduces veiling glare from light outside the FOV (a requirement for some users).A smaller FOV gives sharper edges (less curvature).A smaller FOV, especially in a porro, saves weight by permitting smaller prisms to be used to deal with a smaller light cone.It's set by the acceptance angle of the eyepiece so it depends on the trade offs you make in the eyepiece between all the parameters of the EP: AFOV; ER; field curvature; astigmatism; lateral color correction; baffling and edging; optical complexity; weight; size. Then the overall design of the bin (the magnification, prism size and baffling in the rest of the bin) converts that acceptance angle (AFOV) into the actual FOV. The optical designer will vary the design to get various desired features of the product specification and budget. Birders are only one target market (with a particular set of favored properties) that doesn't match all the other users.These internal focus porros don't share the same optical design. The Minox actually has it's own design of EPs that differ from the Vixen/OEM design.I think the constraint in this case is the enclosure and prism size and internal focusing mechanism with a negative lens. In a roof you can put that focusing lens much closer to the compact roof prism. I suspect the size of that lens limits the light cone before the prisms and limits the overall AFOV.The small light cone also means the prisms can be smaller (which they are in this bin).I actually think that if the bin designer was a bit more radical they could avoid some of these problems and make wider FOV internally focused immersion proof porro if they made it an "over/under" design (like the Avimo/British Army L12A1 but even more so .. like an old Bushnell ELite porro). That way they could keep the focusing mechanics out of the light path and closer to the face of the first prism. They would also get the objectives closer together and so get the "roof illusion" (another feature against the porro).<a href="http://www.holgermerlitz.de/six7x40.html" target="_blank">http://www.holgermerlitz.de/six7x40.html</a>The marketing people would roll there eyes if you said that. They need to determine how many people will buy the bin, how much it will cost to design and then tool for production and then how many they will sell.It's clear from the Nikon SE that not enough people will pay a lot of money for a premium porro. I think some might pay for a good, inexpensive porro. The problem then is how many of those people would have bough one of your more expensive roofs instead. The goal of business is to optimize profit not to build what the customer wants (as sometimes, as in this case, the two goals don't align).</blockquote>

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