You have a bag full of lenses, but you need a pair of binoculars, too. Binoculars are a true "crossover" accessory that are useful when you're out taking pictures--but are equally useful when pursuing other pastimes, like watching your kids play soccer, or checking out the birds that frequent your back yard feeder.

Externally, binoculars look pretty simple, and they are to the extent that they have only one application: they make distant objects appear closer. They also look pretty much alike, at least on the outside. But don't be fooled by their utilitarian appearance and drab cosmetics. There's really a lot going on inside.

Like any precision optical instrument, the type and specifications of the binoculars should be matched to the application. Boaters and birders have different requirements than photographers. Depending on your other hobbies, you may be interested in checking out waterproof glasses, binoculars with Image Stabilization, internal ranging compasses, wide angle models, and so forth. The good news is that most photographers need only a basic pair of binoculars, and making a smart purchase decision is easy once you get a handle on a few of the differentiating features and specifications.

Understanding The Numbers
Binoculars are generally classified by their power and objective diameter, two specifications that always appear together with an "x" between them. For example, 7x35 are seven power glasses with a 35mm objective diameter. The power number indicates how many times closer the subject will appear. Seven power makes things appear seven times closer. Eight power, eight times closer. It's that simple.

The size of the objective diameter determines the amount of light that can pass through it at a given power. A pair of 7x50 binoculars is brighter than a pair of 7x35 because more light can pass through, all else being equal. When comparing glasses of different powers, divide the power number into the objective diameter. For example, the theoretical brightness of 7x35 and 10x50 binoculars is the same: 35 divided by 7 equals 5, the same factor we get if we divide 10 into 50. Remember, these are mathematical comparisons. Because of differences in lens coating, prism material, and other factors, actual performance may vary.

Higher power glasses are not always better. Binoculars in the 10x range and higher can be very difficult to hand hold. Imagine looking through a microscope during an earthquake. If the image jumps around, that leads to eye strain and headaches, and means you'll probably leave the glasses at home. Stay on the low side in the seven or even six power range but get a pair with the largest objective diameter you can find. The larger objective diameter will deliver a brighter image in dim light.

A good all-around choice is a 7x50 glass for normal field use--but these tend to be large and on the heavy side. Use this configuration as a baseline and prepare to compromise. Small pocket binoculars tuck easily into the outside pouch of a gadget bag or backpack but are not as bright as a full-size pair. Find the size that's right for you--then work on the specifications.

Terms And Applications
Most binoculars use a center focus system, whereby both optical tubes are adjusted by one knurled ring located between them. If you think you'll have frequent need to use the glasses with only one hand, look for the "rocker style" focus mechanism that can be operated with one finger.

If watching stadium sports is high on your list, be mindful of this specification: field of view at 1000 yards. High numbers indicate that the binoculars are wide angle. At a given power you'll be able to see more edge to edge, and that's beneficial at football games and auto races, or when following wildlife that moves into and out of your field of view.

As with camera viewfinders, eye relief describes the distance the binoculars may be positioned away from the eye and still be used comfortably. If you wear eyeglasses, look for eye relief numbers in the 18-21mm range. Also look for Diopter correction with click-stops. Diopter correction allows you to adjust the binocs to match your eyesight. Having click-stops allows you to reset quickly when your spouse borrows the binoculars and changes the setting. Clicks also make it harder to move the dial accidentally.

If you see a pair of glasses marked nitrogen purged, that means the individual optical tubes have been flooded with nitrogen gas and sealed, locking out water vapor that could condense on the inside glass surfaces and render them temporarily unusable. The added benefit is that sealing the nitrogen in seals the dust out. Binoculars with this feature are usually designated fogproof.

High-quality binoculars should feel snug and firm--just like a good camera lens. Having BaK-4 prisms is one of the hallmarks of truly high-quality binoculars. This is a more expensive prism type, but provides sharper images with less eye fatigue. Similarly, better models are phase corrected. When an optical beam is split by a prism or other means, the two light paths can become out of phase. Better quality binoculars use special lens coatings or other methods to correct this phenomenon.

A Photographer's Binocular Sampler
From Olympus, the Tracker 8x25 PC I compact porro-prism binoculars combine compact shape with striking aesthetics. Minimum focus distance of about 8 ft means they're useable at close quarters. They weigh nothing--just over a half-pound--and they'll slip into the tiniest gadget bag as easily as they'll fit into even the tightest budget ($59, street price).