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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1756527" data-attributes="member: 68323">Me too. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />I didn't say that ... clearly there are. For example, if the prisms aren't big enough you have to limit the FOV as would be the case if you decided to add 40mm objectives with the right focal length for 8x magnification. The light cone would be too big for the prisms.They wanted to keep one enclosure/prism/ocular design so had to stick with a single exit pupil size rather than the other way around. So they picked one that would work with all of the magnifications they were planning  along with other constraints tp the design (e.g. weight). They could have chosen 5mm exit pupil but that would have made the family 8x40, 10x50 and a 12x60 which clearly would have been too heavy. The 10x50 were already out of style when designing these bins but the 10x42 was the primary bin (and was introduced first ... the 8x and 12x came the next year).You can see this effect in families that have a 8x40 and a 10x50: the FOV of the 10x bin often suffers more than you might expect.</blockquote>