Just back from ten days in the Rockies with the Zen 8x42 Prime and they performed flawlessly. In the field, in actual use, they’re as good as any alpha in my opinion. Under controlled A/B conditions maybe you can find a few small things to niggle about. But the birds don’t wait and I don’t think I once regretted leaving the 8.5 SV at home. This was a bit surprising to me, because I am (ahem . . .) picky. Best lifer: American Three-toed Woodpecker. Runner up: Williamson’s Sapsucker.
I often wanted something lighter though. My wife has sort of commandeered the 20.5 ounce 8x32 SV which is my personal favorite. The 29.6 ounce Prime is not first choice for an unacclimated 12 mile day hike at 9-10.5 k ft. (insert huffing-puffing smilie here). :-O Where I live I top out at 2200 ft. Well, we had a great day anyway.
Would something substantially less expensive than the $610 Prime do as well? Hmm, maybe, maybe not. I have the 8x43 ED2 and I think the Prime is a solid step up. I would take the Prime over the ED2 in a heartbeat. The Prime flatfield eyepiece is still a little “fussy” compared to the SV, however. Perhaps spherical aberration of the exit pupil is involved? It reminds me somewhat of the 8x32 SE in that regard. Not as touchy though, just not “slap ‘em on your face” easy like the SV.
Regardless, I had a great time with the Primes. They are plenty good enough. You can chase "diminishing returns" if you want to. Your call. But the Primes should thrill any birder. I love ‘em. :t:
Mark
PS: the weight issue should be kept in perspective. The 8.5 SV weighs 29.2 ounces (829 gm) so it's basically a tie. The Prime does feel a little heavy in comparison, so other factors must be involved (open/closed hinge, etc.). And in either case, if you're climbing mountains above the treeline, please take something smaller than a full-sized binocular.