What Is Best Way To Find F-Number For Lens/Camera?

Is hard to get to actual number quickly?

F number is listed on essentially every manufacturer spec sheet.

  1. Manufacturer known to lie someoftimes.
  2. How would I tell if they lie about f-number?
  3. Why would they not lie, if I can not tell?
  4. Thenfore, how do I tell 'actual' number?

I buy this one while back when. Here is "quoted" spec. I multiplied/divisioned all digits, in all thought of ways, even tried for lucky numerator jumbling. Never got the low boasted F1.2!

Suspicion Note: also if u try to take lens apart to find out size secrets, watch how you will never never, get it back work again, even if you fit it back the way it looked before.