In thread 2 of "eye relief - Rule of thumb method?"
Pete Gamby said
"We measure from the outmost/top edge of the eyepiece in its lowest position (twisted down or folded down) to the point at which the exit pupil diameter is measured to be at it's maximum/optimum e.g. 5.25mm for an 8x42 binocular. This is done using a device with an extendable tube which projects the exit pupil onto a semi-translucent sheet with a scale.
HTH
Cheers, Pete"
I find this method more useful, as an eyeglass wearer, than the technically correct measure but it would also be useful to know the axial distance from the rim to the apex of the ocular.
I only have my BGA Classic to hand. The eye relief listed on their website is 19mm. That corresponds exactly with the lens to ER measurement. The distance from the rim of the eyecup to the ER is 16mm. I actually use an O-ring to reduce it about 13.5mm which is comfortable with my glasses. I'm pretty sure they list true ER rather than available ER for most of their models.
Of all the binoculars I've reviewed I've only found one occasion where the manufacturer used the available ER. In fact the 14.5mm listed was actually quite generous with glasses, but I'm sure it would have discouraged a number of potential buyers.
David