So, as my marine binoculars are Opticron Pro Series 2 7x50, and individual focus, does that make me not a 'serious birder'? Just curious...
You will not find an argument from me on whatever you use for “serious” birding. If you derive enjoyment from it, I’m ecstatic!
I will say, however, that MOST serious birders wouldn’t dream of using an individual focus bino, unless they were dedicated to shore birds at a distance or raptors in equally distant crags or rock crevices in which focusing speed would not be critical.
The Opticron Pro Series II/Swift Seahawk/Bushnell Navigator/Fujinon AR, or other models from the same OEM, while not the “Alpha” quality that some get twitterpated over, are robust, truly waterproof binos to last a lifetime. In addition, those large oculars—as on the lower version of the Swift Seahawk attached below—are very nice.
Still, if you are trying to follow a bird in flight, or rapid movement of any kind, at a distance close enough for a 7x bino to be of use in identification, an IF bino can be very frustrating.