Below is what I wrote in response to a thread on the NLs in the spotting scope forum, before realizing the thread was in the wrong place. From my experience comparing the 12x42 NL to the 10x42 SF, I'd expect the 10x42 NL to be a bit better than the SF, but it's a very high cost to upgrade from the SF to the NL for only a marginal performance difference. I really like my SFs, so I got the 12x42 as they're giving me something a bit different. I enjoy owning both. If I was starting from scratch or could only afford a single pair of alpha binoculars, then the 10x42 NL would probably be my preferred solution (assuming that optically they're at least as good as the 12x42). With the 10x42 NL, I think you'd have pretty much everything you'd want to do with a handheld binocular well covered, except for having something small and cheap enough to just throw into your travel bag without thinking about it (for that purpose I have a pair of 8x30 Monarch HG I bought refurbished direct from Nikon for a bit over $500).
I only have the 12x42 and have no experience with any of the others. I've seen reports from others on Birdforum saying that the optics in the 12x42 are a little strained compared to the 8x42 and 10x42, but if that's the case, I doubt it is a difference my eyes could see, as the image quality is really excellent. I've been comparing them to my Zeiss Victory SF 10x42, and I whilst I'd pick the 10x42 first for general use that would involve low-light viewing or nearby objects like birds in undergrowth, I really don't feel that the NL 12x42 is giving away that much in terms of brightness. That said, I don't feel my eyes are flexible enough to get the most out of a binocular with a very large exit pupil. All my binoculars have exit pupils in the roughly 3.5-4mm range - 8x30, 10x32, 10x42, 12x42, 15x56, 25x100. I've tried Zeiss 8x54s in the past, but really didn't feel they were giving me much more in low light than my SF 10x42s.
I got the 12x42 because I feel they're offering something different from all other models out there - a 12x binocular that is as field-usable as a good 10x42. I honestly don't think there's a better binocular for long-range, hand-held, daytime viewing (at least for me). But they're still good for other things (i.e. they're not a dedicated long-range binocular, they're just not the very best when absolute field of view or brightness is the main consideration). If you're happy with the other binoculars you have, it seems the 12x42s would be your best option. If, however, you're thinking of buying NLs to replace one or both of your existing binoculars, then it's a different calculation.
