Occasionally a glass comes along that makes me question my dedication to 8x binoculars. The 10x LXs reviewed in this issue of BVD are such glasses. So are the 10x50 Leupold WindRivers.

One of the major drawbacks of 10x40 or 42 binoculars for birding is that the smaller exit pupil (the circle of light coming out of the eyepiece: actually the image of the objective lens created by the eyepiece) is considerably smaller than that of 8x40s or 42s. A smaller exit pupil is harder to keep centered over the eye, or to keep the eye behind, and that adds, again considerably, to the difficulty of using binoculars in the field, and to the consequent strain and fatigue.

Another factor, of course is the difficulty of holding a 10x image still enough to see. It is a matter of leverage. I always ask people to imagine that they are being asked to put a key in a keyhole using a pole 8 times the length of their arm, and then to do the same task using a pole 10 times the length of their arm. Which is going to be easier? Which pole is going to magnify the inevitable unsteadiness of your arm and body more? Which pole is going to be more susceptible to vagrant winds? If you have to do the same task 100 times in a row using one pole or the other, which is going to tire you out faster? Your mind is continuously doing something very similar when it attempts to assemble the image seen through hand-held binoculars. Our bodies are never still and a 10x glass simply magnifies the motion more than an 8x glass and makes it harder for our brains to sort of what we are actually seeing.

One way to compensate for the smaller exit pupil problem is to just up the size of the objective. 10x50 binoculars have the same exit pupil as 8x40 binoculars and should be as easy to see through. Of course the larger objective does have additional advantages. All other things being equal, a 50 mm objective will deliver both more light and more detail to the eye than a 40-42 mm objective. Its just physics. It might seem then that you could get an exceptionally bright, sharp, easy to view image with a 10x50 glass.
Unfortunately most 10x50 glasses are so much heavier than 8x42s that the extra muscle strain of carrying them all day and holding them up to the eyes quickly offsets any advantage you might have gotten from the larger objective.

And then, along comes a glass like the Leupold WindRiver. Leupold has, apparently, taken the very successful optical design of the Pentax 10x50s (reviewed previously) and pared the body down to the bare minimum to hold the lenses apart and keep them aligned. The WindRivers are slim and light, well balanced, and easy to carry and hold. They have only a 5 degree field of view (262.5 feet at 1000 yards), but their eyerelief (and the excellent screw up, screw down eyecups) make the full field usable for any birder. At 10x the 5 degree field feels completely adequate.

The view through the 10x50s is everything you might expect. Sharp, bright, superbly detailed, and the equal of any view you can buy today (with the exception of the Nikon Superior Es, and allowing for, as you must always when considering roof-prism glasses, especially mid-priced roofs, an added amount of chromatic aberration).
The Leupold WindRiver 10x50s are binoculars I could easily be persuaded to carry as my full time birding glasses, and binoculars that showcase all the advantages of a high-power glass with practically none of its drawbacks. Impressive indeed!