We birders are constantly focusing and refocusing our binoculars on the birds around us--a wren a mere 15 feet distant one moment, and a hawk soaring above the ridge the next. In fact, a significant amount of our observation time is devoted to focusing.
Several systems achieve focus in binoculars today, and all of them can be used for birding to at least some extent. But if you plan to use one pair of binoculars for all your bird watching, you need to give some thought as to which system will best bring your favorite species into focus.
First, there are the so-called "no-focus" or "focus-free" binoculars, to list just two. These binoculars have no center wheel or movable eyepieces--there is nothing to adjust or focus at all. You simply look through the binoculars, and everything is supposedly in focus all the time, no matter how far away. It's a miracle! What will they think of next?
There is really nothing new here at all. Focus-free binoculars are nothing more than a cheap version of the old individual eyepiece focus design, with the focusing feature removed to keep the price down. Essentially, these binoculars have been permanently focused at the factory.
Read the fine print. These "miracles" are limited to people who have perfect 20/20 vision and eyes of equal strength, since there is no way to adjust them at all. Worst of all, "no-focus" are really "can't-focus" binoculars for distances much under 30 yards.
Last, these are exclusively bargain instruments--prolonged use is sure to put you on an eye doctor's Christmas list.
If you really want that "one shot, everything beyond 30 yards is in focus" feature, you are better off with the original system. This is known as individual eyepiece focus, and it has been used on binoculars for a long time. With this system, you focus by turning each eyepiece individually until an object at some distance (at least 100 yards) is in focus. Thereafter, everything beyond 30 yards is always in focus. There is no center wheel to move or further adjustments to make. And, unlike cheap no-focus binoculars, individual eyepiece focus binoculars can be adjusted for your eyes, no matter what your vision.
Because these binoculars have few moving parts, they are typically rugged and durable, and easy to seal against moisture and dust. This makes them a favorite for marine and military binoculars. Most are Porro prism binoculars, but there are a few roof prism models on the market as well. Either way, excellent optical quality is available, and most models are sharp enough for birding.
Individual eyepiece focus binoculars sometimes get a bad rap for eye fatigue. Much of this, however, is caused by the observer who constantly squints or is too lazy to check the focus setting occasionally. With this type of binoculars it is particularly important to observe with the eyes totally relaxed. This may take some practice, but it is a good technique to learn with any instrument.
Individual eyepiece focus is a serviceable system for birding, provided birds are in the no-focus zone beyond 30 yards or so. There is certainly nothing faster when it's time for action--just grab your binoculars and observe. And it's a real treat to always be in focus on passing waterfowl, hawks, and other birds in flight. It's also the ideal system for scanning open country in search of distant hawks and eagles.
Conventional center focus binoculars are the best choice for an all-purpose birding glass. This system, with its familiar center focus wheel (or rarely, a lever) will allow you to focus quickly and surely on any bird, regardless of the distance, as long as two conditions are met.
First, the particular center focus model that you choose must be able to focus on objects as close as 25 feet away (15 feet or less is much better). This sounds strange, especially if you are new to birding, but much observing is done at short range when birds are in thick cover or at a feeder. Thus, minimum or close focus is an important feature in birding binoculars, so check before you buy. If your dealer does not supply close-focus information for various models, you can estimate it yourself.
Simply crank the focus wheel to its minimum setting and pace the distance to the point where the floor or some object first comes into focus. If the binoculars will not focus closer than 25 feet, and this includes many models, try a different pair. Even 20 feet is marginal when you are in close cover observing wrens, kinglets, vireos, and other small passerines. Binoculars that do not focus at least this close are not engineered for birding.
The second feature to check on a center focus model is speed and sureness of focus. This is somewhat subjective, but a well-designed center focus will bring a bird sharply into view with reasonable speed and a minimum of fuss. You will know exactly when the binoculars are in focus; there should be no hesitation or guessing. Avoid any model that is slow, sloppy, or fussy to focus.
I prefer binoculars that I can focus and operate with one hand if necessary, because I often work with other equipment such as cameras and spotting scopes. It's also a nice feature to have in tight quarters such as a blind or a vehicle. For this kind of activity, the slim profile of roof prism binoculars is ideal. Even in large instruments, this design typically places the index finger right on the focusing wheel when grasped with one hand (don't forget the strap).
Many binoculars, especially Porro prisms, have the focusing mechanism exposed. These external systems have a visible "bridge" that moves the eyepieces up and down as you turn the focus wheel. This time-tested system works well when made of quality materials, but requires some care and maintenance because the working parts are exposed. Keep them clean.
Internal center focus systems house all the focusing parts, except the wheel, within the body of the binoculars. This provides more protection for the focusing mechanism and makes it easier to seal and waterproof. This is the preferred system for really severe conditions. The closed mechanism also discourages tampering--a more frequent temptation on the visible parts of an external focus glass.
As a final note, be sure to check ease of focus with gloves on if you bird in cold weather. The focus wheel, especially on some compacts, tends to disappear when you wear gloves.
Whatever your chosen system, keep your eyes to the sky, and stay focused.