What, exactly, does the "HD" designation describe? In TV, etc. it relates to a very concrete metric--pixel count, essentially--which clearly doesn't apply to binoculars.
Anyone have a clue?
I'm wondering if optics company do?
It depends on who's using the letters. For the Leica Ultravid HD, the company is saying that it means three things: (1) redesigning the housing to cut down on stray light, (2) increasing the light transmission by 3 %, and (3) using fluoride (FL) lens glass.
Source:
http://www.bestbinocularsreviews.com/leica-binoculars.php#hd_lenses
In the case of the Bushnell Ultra HD, they are using the letters to designate their "RainGuard® HD water-repellent lens coating," which besides repelling water, "averages 7.25% more light transmission than Nikon Monarch across the entire visible light spectrum" (so you know whose business they are trying to cut into, the very popular Monarch series). The Ultra HD also has ED glass, and that's a bigger deal, particularly on the 10x42 model since CA goes up with magnification.
My take is that "HD" is a marketing term rather than a technical term like it is for "High Definition TV". Optics manufactures can make it mean anything they want, but in general, it's another name for bins with "ED" glass.
Some other names are "XD" (Vortex), "FL" (Zeiss), and
"L" (Canon 10x42 IS bin). The latest entry into the field is the Theron Wapiti APO-ED, which uses "APO," meaning it has has better correction of chromatic and spherical aberration than common achromat lenses.
http://en.wikipedia.org/wiki/Apochromat
Since that term is used a bit differently with "APO" telescopes, some take exception with its use in bins, whose shorter focal length make it impossible to eliminate all false color (apochromatic = without [false] color). So it becomes yet another marketing term.
Since no company to my knowledge, except Minox, states the exact type of extra-low dispersion (ED) glass they use, there is no way to compare different bins with extra-low dispersion glass on paper. Even the terms "Flourite" can be misleading, as Henry explained here (post #9).
http://www.birdforum.net/showthread.php?t=187425
So take it in general as an improvement from the normal glass lens bins in the companies line up, but again, beware that it's a marketing term, so comparing two bins with the "HD" designation is not necessarily comparing apples to apples.
Brock
P.S. I wrote this before seeing Kevin's post. Being more long winded, and having to check references, he beat me to it (not the first time either!).
P.P.S. Welcome to Birdforum!