So I got the Zeiss HT 10x54 and writing my first impressions without going into much objective analysis. I compared it to the first generation green-gray 8x42 SF which I also got fairly recently. I know it's not a direct, fair comparison given different classes of optics but FWIW.
Having done a bit of research about the 10x54 HT, I was warned that the field of view was sharp only in about 70% of the entire view. I can't confirm that but even if that's so, it's an irrelevant fact. Because I am looking at objects in the middle of the picture and at no point does it matter what's on the periphery of that picture. You point the device at what you want to see and it's razor sharp. The entire field not being sharp is not an issue.
I looked at the same things side by side with the SF and the HT and a few things became obvious. Actually I did a bit of birding today and looked at squirrels. It was easier to find the animal with the 8x42 but once I found it, it was more enjoyable looking at it through 10x54 HT.
The field of view in HT is noticeable more narrow. The field of view of SF is noticeably huge.
The depth of field of HT is noticeably shallower. I had to use my focus a lot more.
The focus in HT was magnificent. Butter smooth and no issues. The focus in SF was a bit resistant but I equally had no issues with it. They do appear to be much different. The focus speed of HT was just right. The focus speed in SF was a bit too fast and I tend to overshoot a bit and back down. It was harder to dial to precisely what I wanted with SF -- kind of, but not really because the depth of field was much better. Still the focus is better in HT, I think. If looking at infinity, in SF I just dialed in all the way to the right until the focus wheel could not move anymore.
Which brings me to the next point. Diopter value at infinity. Zeiss doesn't publish that info. I don't have hard data on either SF or HT but with SF, I am maxed out at several hundred meters. It is sufficient but just barely. Given my eyesight is at -5.25 diopters I estimate that SF has a diopter value of 6D at infinity. The HT had a bit of overdrive left at infinity, I estimate that it has 7D at infinity.
Weight. SF is feather-weight. I still can't believe how light and compact it is after using it for several months. It feels like it's made out of paper. HT feels twice as heavy but it's not as heavy as I thought it would be, nor as bulky. In reality there is about a 10oz weight difference. The ergonomics of HT is better than I expected the dual barrels are very comfortable. I think the 10x54 configuration is a true winner. Ergo of SF is A+.
Eyecups. HT feels just right with both eyecups fully extended. Officially it has 16mm of eye relief. Unofficially I would estimate a bit more, maybe 18mm. SF has more ER than HT. I think at least 20mm. This is how it "feels", not what the books say. Eyecups in SF fully extended are too high, I have to crank them down a bit and that causes an issue because they do not stay up in that position. They do while using them but somehow over time they gradually lower themselves.
I have no issues with optical performance with either HT or SF. Both are impressive optical instruments but HT is doubly so. It's what you take when you are really serious and you need the best. Most power and most resolution you can realistically handle in the field. I can't picture another device than would exceed HT because it does everything perfectly. The 10x power does not shake as much as I thought it would. I can manage it off-hand but it seems like 10x is at the top of what I can handle. I am pretty sure I could not deal with either 12x or 15x. I do not know if it's the bulk of it or the 54mm objective is what keeps it stable.
10x54 HT is not an non-enjoyable experience. However I would not say it does not benefit from being stabilized by something, either a tripod or just leaning against something, or resting on something. But then so would 8x42 SF. They are much closer in terms of 'shake' than I thought. Yes, you can hand-hold the 10x54 HT for extended periods. And yes, 8x42 SF is also pretty easy to hand-hold for extended periods (obviously). All in all, it's not a dichotomy, it's not 10x54 versus 8x42. Having said that, I never used 10x42 or 10x32 so have no idea of weight/bulk or a 50mm+ objective or a larger exit pupil size contributes to stability.
Both HT and SF generate that 'wow' feeling. HT I think generates a bit more of 'wow' but it could be because of the new factor. It can definitely punch farther than SF given 10x HT is 25% more power than 8x SF. Zeiss HT resolves more and sharper due to being 10x. It can reach where 8x SF cannot. The FOV shirinks but so what. You don't need a huge FOV, you need the item you are looking at sharper and in closer focus. Unless you specifically need huge FOV such as when panning.
However the SF also has a considerable 'wow' factor but it's not so much the power of 8x but the stunning and huge and edge-to-edge sharp FOV. Not that I can take advantage of it but it's there. The other 'wow' factor is the weight and compactness. I don't know why anyone would want to get anything smaller than 8x42 SF since it's already pretty compact and as light as it gets, it's the optimum size / performance ratio IMO.
Things I did not like about the HT:
Nothing optically. But the armor is weird, it is good on the sides but on the inside it has that slippery polymer that leaves fingerprints, hair sticks to it, dust, etc. When it gets wet, it gets slippery. What was Zeiss thinking. They could have made the outer type of armor wrap around the entire binocular. A la Swarovski EL, which I felt had superior armor that looked new after a decade.
The eyecup cover constantly slips off, it has almost zero tension. So does 8x42 SF eyecup cover. Weird.
Things I did not like about 8x42 SF:
As mentioned above, the eyecup cover slips off. The focus is a bit stiff, a bit too fast. Not a big deal, I got used to it. The huge depth of field saves this one. The eyecups on gen 1 retract too easily and lack settings or clicks. They could have done a better job there.
I haven't noticed any chromatic aberration with either but then I did not look for it.
Things that are an unexpected surprise:
Both HT and SF have hydrophobic coatings. You should never clean the surface with a microfiber cloth, it is enough to just shake the water off. Nice feature that my previous Alpha did not have to the same extent, namely Swaro EL. (pre-SV).
In terms of birding. I think 8x42 SF might be the more practical, everyday everywhere-carry type of device versus 10x54 HT. It's compact and light and is great for scanning the terrain to find something and it does 80% (at least) of what HT can do, the important 80%. Once you find something, ideally, you can switch to 10x54 for better identification but I am not sure than 10x54 is the greatest single it-does-it all device. Maybe but I am not sure. I wonder if 10x54 HT is the better astronomy binocular versus Zeiss 8x42 SF. You really need the greater power of 10x versus 8x for astronomy viewing, it makes a difference, and this becomes especially obvious if you rest it on an object for stability, improvising or using a tripod.
Made stable, 10x54 really shines. That is where 10x54 really spreads it wings. This is not to say that you cannot employ 10x54 without rest for astro viewing, you can almost as successfully as 8x42. So 10x54 has fewer disadvantages versus 8x42 SF for astro viewing IMO. I feared that 10x54 would have unmanageable shake making the additional power useless. It doesn't.
I haven't been able to determine yet if the HT does better in twilight versus SF. Theoretically it should and it does seem a hair brighter but need to do more tests. I feel that either 8x42 SF or the 10x54 HT could serve as the primary and only binocular for everything ranging from birding to astronomy. It's hard to say which is better. They are different. They intersect in a huge portion but there are things HT does better than SF and vice versa.