Try this masterclass from my pal Cameron Cox. He knows a thing or two about Optics too! I share his views...A post on Birdforum discussing Leica and Ziess 7x42's...
"First let me make it clear that I work for Leica. I am, however, in an excellent position to answer your question as I have used the 7x42 Ultravid HD everyday for almost a year. I also worked as Morning Flight Counter for Cape May Bird Observatory a couple of falls back, a project sponsored by Zeiss, and used the FLs all-day-every-day for three months. I’ve also compared them side-by-side at a number of birding festivals.
In choosing between these two binoculars you are choosing between superb and extraordinary. Meaning that there is not a bad choice here. Either way you are getting an incredible tool that will serve you well for years. In my opinion if you want to bird like your hair is on fire, 100%, all the time then you should be using one of these two 7x instruments. If size/weigh are an issue or other pursues such as butterflying or odeing also capture your interest then you can consider other options, but for the serious birder you are in the right place.
I’d agree with other members that the feel in the hand is an important aspect of this decision as they feel quite different. Only you can assess what feels better for you. Personally I prefer the balance of the Leicas and the feel of the more rounded eyecups against my eyes.
I’ll also agree that the different in FOV is apparent. It isn’t huge but you do notice it.
The area where I believe Leica has a significant edge is durability. Both binoculars have excellent warranties, but the best warranty is the one you never have to use. Leica has the highest degree of waterproofness of any company that lists their specs (to 5m). This speaks to the care that goes into sealing the Ultravids and to Leica’s use of all metal body. Those seals are particularly important when birding in the tropics. Internal fogging is the last thing you want to see once you’ve plunked down your money for a trip. The Ultravids also receive a 100G shock rating.
If you have the opportunity at a field trip, convention, or birding festival, ask other birders with high-end optics how they feel their investment has held up. Has the rubber armor bubbled? The eyecups broken? Plastic pieces fallen off? Ect.
Color rendition is another difference that is apparent to many people that try both of these excellent binoculars. To me this is a very striking difference but it may not matter to you. I’m sure engineers from both companies would go to the mat swearing that their optic provides the most accurate color, but it comes down to what the individual user prefers. Personally I prefer the color rendition of the Ultravid. Others like the Zeiss. I won’t describe the differences for you. Take a look for yourself. If you can see the difference then decide which you prefer, if you can’t see the difference then it doesn’t matter.
As to the question of sharpness, it almost seems that whichever binocular an individual expects will be sharper when they pick them up is the binocular they perceive to be sharper. When comparing them side-by-side most people see little difference in sharpness, while some decidedly see the Leicas as being sharper and other perceive the FLs as sharper. One way or the other, in the field there will never be a situation were the difference in sharpness will cost you a bird ID. To me that is always the bottom line.
Both are great. Some dealers will let you buy both to try for a week then send the one you don’t want back. Testing them side-by-side is by far the best way to make this decision. Often someone immediately gravitates to one or the other for very subjective reasons that may have never entered their mind until they tried both for themselves.
Cameron Cox, Florida
p.s. Just a thought from yours truly. Have you ever noticed (I'm sure you have) if you've a zoom eyepiece, that the brightest, sharpest image is on the lowest mag'. In fact I leave mine (Leica APO 62 with 16x48 eyepiece) on lowest mag and therefore widest FOV most of the time, just bumping it up when required.
As far as I'm concerned you'll know (if you've followed for a while) that I prefer a wider, brighter image. If something is further away...I'll use my 'scope!