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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1654741" data-attributes="member: 68323">As bins tend to keep a constant f number (around f/4 usually) the focal lengths tend to go down as the aperture goes down. That also means the ER tends to go down too if you just scale a bigger bin to the smaller aperture.Of course you can mitigate this by say increasing the f number and making the bins proportionately longer. Or change ocular lens design so you get a larger ER. All extra trade offs for the smaller aperture.The other thing is making sure one is comparing Apples and Apples with ER. Some give the "distance from the back of the last ocular lens" but then don't specify how much effective ER you loose in the eyecup or recession of the lens (to protect eyeglass lenses). 2 or 3mm is typical. And that's the difference between the tvwg.nl and the manufacturers specs. Others manufacturers seem to be a bit more generous. For example both my 42mm and 32mm FL quote the same ER at 16mm and that one seems to be a bit generous (I have one eye that needs less ER than the other so I notice a difference between the eyes). And it's clear for me (a medium myope) that the SE 17.4mm tends to be a bit too much.</blockquote>