Serious question about the Leica UVHD plus, how exactly is the Zeiss SF optics better or a step up, is it because of a flatter field, the larger apparent fov, better ca control, do they have a higher resolution or better contrast than the Leica, what about ease of view or dof, just curious as to what exactly is better in the SF image/optics that makes it superior in your opinion. I haven’t tried the SF yet but it or the NL will be my next purchase.
Casscade owning both the UVHD+ and the SF in 10x42 I can share:
1. The Zeiss is slightly brighter (though the Leica is no slouch).
2. The Zeiss controls CA significantly better. Looking through the SF, minor CA only visible around the edges / the outer 20% of the circle to me (although I have yet to come across a bin that eliminates it completely). This was a significant factor in the purchase for me, as I'm a bit sensitive to CA and the one thing that always bothered me about the UVHD+ is that CA is strongly visible in the middle and throughout the field.
3. The SF has a flattener as you mention, which is great if you ever decide to take photos through the bin. Baring this, the inclusion of a flattener manifests as a slightly sharper, crispier image overall, but for general birding use it's probably not a huge deal as your focus is really on the middle of the image - the Leica is reasonably sharp in the middle as well, but I believe the added CA has an effect of sort of slightly softening / blurring the image relative to the SF.
4. In terms of contrast, the SF offers a slight improvement but this may simply be attributed to it's greater overall brightness (just a guess).
5. FOV is again very slightly better in the SF (caveat - I only tested it between 50-200 meters). The difference is noticeable, but not dramatically so as it amounts to around a 15% increase in area. The difference was much more noticeable on the NL.
So there you go - apart perhaps from the improved handling of CA, none of these improvements in isolation would have me shell out the extra £/$ for the SF (particularly given the best-in-class fit & finish of the Leica), but taken as a whole it's a different story - looking through the SF does feel like a step up. Since you mention the NL, I can tell you the only difference I found between the NL and the SL (optically) was the added FOV. The NL also felt a little better in the hand due to their oddly shaped barrels. And I thought the headrest was a great idea as it slightly reduced the shake at 10x. Overall these are both the best bins money can buy. My decision to purchase the SF simply came down to the fact that the NL's were running me £2,500+ (headrest included), whereas I was able to find a like-new SF for over a grand less. Cost no object, I would probably buy the NL's - but it's a very steep premium to pay for a slightly wider view and a headrest. Had I been dead-set on the NL's, I might wait a couple of months for the price to come down a bit from the stratosphere.
To me, a more interesting comparison might be between the 10x42 UVHD+ and say a 10x42 Conquest HD, as you can buy a mint-condition used UVHD+ for the same money as a new Conquest. Having compared both, I can tell you that the optics are
very similar, with a slight edge in terms of CA control in favour of the Conquest. However, the Leica is built significantly better - phenomenal eyecups, awesome flip-down lens covers (versus the irritating rubber Zeiss covers), best diopter on the market in any price range, killer good looks and premium feel). For someone in the market for new bin under $1,000 / £1,000, I would take a very hard look at these two. Your choice would come down to slightly better CA control vs outstanding feel / ergonomics (a difficult choice). For people that simply just want the best optics for the money, a gently used Conquest is pretty tough to beat. But at the price of a new one, a used UVHD+ is very stiff competition indeed.