To see game in dim light or watch songbirds in the shadows, you'll want a 5mm exit pupil, maybe 6. So for a 7x binocular a 35mm objective is about minimum. Want lOx magnification? To get that 5mm exit pupil, you'll need 50mm front glass.
When I'm not toting a rifle, I'll strap on a harness to carry 10x50 Swarovski or Leica binoculars. The higher magnification gives me more detail, and bigger front glass ensures plenty of light transmission in the shadows. When packing a rifle myself, however, I'll leave these 42 ounce glasses at home. The hills aren't getting any easier to climb. A 50mm binocular all but mandates a harness to transfer the weight from your neck to your shoulders. Harnesses make that weight bearable, but they're annoying when you want to shed or put on a jacket. You must extricate yourself from the straps and set the binocular down. Then, when you're finished adjusting your outerwear, the harness must go back on. Even if the weight and inconvenience don't nettle you, the bulk of a 50mm binocular may. It's not a glass to slip easily into your shirt, and most models are long enough that you'll notice them banging against your rifle.
To be honest, I'm getting perilously close to liking little binoculars. They fit in a big jacket pocket and ride under a jacket front with hardly a bulge. They don't require a harness.
For all their advantages - and a proliferation of the type during the last two decades pocket-size binoculars have become something of a counter-culture in the hunting community. Binocular companies these days hawk image brightness as if all game were shot in the dark. Bigger front glass is promoted as if it could somehow suck animals out of the shadows. The fact is that game can be spotted only when game is there, and you'll glass up a lot of antlers when light are adequate for iron-sight shooting. Toting enough binocular glass to probe the darkest thickets at dusk is like packing a bi-pod tactical scope and target weight barrel on your hunting rifle in the expectation of cross-canyon shots. Where weight and bulk don't matter, performance comes cheap. But if your mobility is compromised by a heavy binocular, it can keep you from looking where the animals hide.
Still, the true compact binoculars aren't bright enough for all around big game hunting or birding. A 9x28 binocular has an exit pupil of 3mm, useful only in very good light. Binoculars with exit pupils this small are like glove compartments too cramped for maps, and jacket pockets too tight to get your fingers in. Like an under-size spare tire, they're better than nothing, but not much.
Optics firms have worked hard to design better compact binoculars.
Some of these have very good lenses. Some are even easy to hold. But in my view, they are not bright enough or comfortable enough in use for people who glass a lot. Hunters and birders who are serious about what they do glass a lot.
With that in mind, there's much to like in binoculars that are almost but not quite compacts: those that weigh 16 to 24 ounces and feature objective lenses of 30 to 35mm. They're available in both porro and roof prism configuration. Largely overlooked in the rush to higher magnification and bigger objective lenses, they may now be gaining slightly in popularity. Properly so.
My last hunt in Alaska turned up a lot of open landscapes. So did a recent deer hunt in Oregon and a pronghorn hunt in New Mexico. On these trips, I carried an 8x30 Zeiss Diafun binocular that looked more appropriate for a football stadium. I took it along because it was lightweight and compact. Despite an exit pupil just shy of 4mm, it was very bright. I was reminded again that a large exit pupil becomes an advantage only in light conditions that seldom occur.
Here are five almost compact binoculars (as shown on the next page) that offer top rung performance in a package not much bigger than a good ham sandwich.