As quoted by Henry Link from a thread on AFOV. The thread link is at the bottom.
"Nikon has just adopted the method described here for AFOV specifications:
http://www.ave.nikon.co.jp/bi_e/prod.../chart_001.jpg
Others may follow (or already have done) in order to conform to the ISO standard. In the meantime Nikon specs will be confusing when compared to other manufacturers. I think it's probably best to continue to compare approximate apparent fields by multiplying the real angular field by the magnification like it's always been done in marketing materials."
https://www.birdforum.net/showthread.php?t=109343
Dennis,
I wouldn't make that last statement now. When that thread appeared back in 2008 I didn't yet realize that the true AFOV, including distortion, can be measured pretty easily with the binocular mounted on a panoramic tripod head with a degree scale. What's the point in advocating for either the ISO or the simple calculation when both are likely to be wrong?
If you look at the current AFOV specs from Swarovski and Zeiss you'll find that they don't match either calculation. That means they're either accurately calculated by including known amounts of distortion or they're measured. Hopefully everyone else will eventually do the same