When does 50 mm + objective become useful?
In talking with Leupold Techs, never.
They sell 50 mm scopes because of demand and a perceived gain.
As to 50mm being needed: All I can say is those who have been down the road and have the experience fully know the answer, and THAT is why the 50 and 56mms sell. Sure, some buy "just because." But if your shot opportunities are few and far between -- and if, like me, you are not mobile and can't stalk closer -- that 36mm objective on a 2.5-8x VX-3 will prove utterly useless on a 175 yard hog in weak moonlight. Some will talk about exit pupil, but you HAVE to have magnification under those circumstances (not to mention a visible aiming point). Trust me. Been there. Done that. I've tried all sorts of scopes that would be up to the task for this sort of usage, and not a single 40mm objective cracks the top 25 in my experience.
There are a precious few scopes which boast both the resolution and contrast to even allow 6x to be a viable option at 100+ yards (in moonlight/very low daylight). 8x usually works fine, but if you can get 10x or more, then all the better. You can crank a 40mm objective all the way down to 3 or 4x and have an impressively-bright image, but I assure you making a humane shot at extended ranges will be out of the realm of possibility. You simply won't be able to discern enough detail. Hunting under moonlight or very low light is a completely different ballgame than shots taken in decent daylight.
