AoV Calculations

I have come across two cameras today where no AoV is given. The IPVM specs say "none". The manufacturers spec sheets don't offer it either. Every camera has a AoV. How can I manually calculate it? Why wouldn't this info be offered on the spec sheet or on the IPVM product page?

Example

http://overseas.hikvision.com/en/Products_accessries_209_i5602.html#prettyPhoto

https://ipvm.com/camera/ds-2cd2522fwd-is

And plugging it in on the caluclator it defaults to 80... but it's not offered anywhere in writing.

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