I raked out my loft and found a 10x50 old 70's bins. They weigh a Ton but are still in good order.
Is 10x50 the right way to go. But i would not want to carry these far!
Sounds ok to me, although those old bins can weigh a ton, try them out and see how they go in the field. Theoretically a 50 mm lens should let in more light than say 42mm, but modern lenses are shaped and coated to cut down on optical aberrations and flare from stray light bouncing around in the barrel of the bins, but old bins that don't have apachromatic glass and modern coatings can be surprisingly good. I met a guy on holiday who used his grandfathers 1950's bins, they didn't seem to hold him back at all.
Jackie
Buy a good strap (Neoprene is excellent) and you can walk with them for miles and miles. This whole weight issue has become far too serious.
happy birding,
Renze