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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1756192" data-attributes="member: 68323">I don't see much evidence for that assertion in roofs. The exit pupil preference combined with a magnification preference seem to explain all the differences.I think there is a difference (as I explained) between the very bottom end where it's clear that Zeiss and other makes are sticking to a minimum exit pupil. So that's a constraint.For the others they have more latitude but it seems to be the case that an exit pupil between 4mm, 5mm or 6mm is certainly preferred for compact and full size bins. Combine that with the different preferred magnifications (7x, 8x and 10x) and you end up with a rather limited set of numbers some of which have the same exit pupil in the same family. Add a desire to keep the weight down for most people and the combinations become even more limited.For example the Zeiss 8x30 and 10x40 I think comes down more to commercial factors. People want 8x bins and 10x bins and you want to deliver them with 4mm minimum exit pupils. If you want 5mm exit pupils then you make 8x40 and 10x50 (though that latter starts to have a problem with weight). So you tend to go for 8x30, 8X40, 10x40 AND 10X50.In the cases like the 42 versus 56mm or 30mm versus 40mm bins I doubt there are many shared components: the case sizes are different, the prisms are probably different (though without pulling a bin apart we can't know for sure) and the oculars are probably different too (different focal length objectives need different oculars).  But sometimes the overall design can be shared e.g. 8x30 and 10x40 Classics is a good example of this with both having the same overall roof prism design with moving objectives. I doubt but I'm not certain they don't have the same oculars as the bigger enclosure in the 10x40 case gives gone a bit more freedom. In that case I think the designers are just trying to cover two markets with a related design.</blockquote>

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