To identify distant birds accurately, a binocular is essential. Two basic designs are available: Porro prism and roof prism. In Porro binoculars, the light-gathering objective lenses are set wider apart than the lenses you look through. For years Porro binoculars set the standard for everyday use, being rugged and less expensive than roof prism binoculars, in which the eyepieces and objective lenses are aligned with each other. Roof prism binoculars generally weigh less than Porro types and are more water-resistant, but they tend to cost more.
A binocular’s name is typically followed by two numbers separated by an “x.” The first number refers to the magnification or power of the lenses. Most birdwatchers use binoculars with a magnification between 7x and 10x. The higher the magnification, the closer the object appears in the field of view – and the more noticeable vibration from hand shake becomes as you try to hold the binocular steady. The lower magnification allows better viewing at close range (watching warblers, for instance), while greater magnification works better for picking out birds from a distance. For general-purpose viewing, binoculars with 8x magnification are becoming the choice of many birders today.
In general, the more light transmitted by your binocular, the better you will be able to see birds. Here’s where the second number on the binocular (the one after the “x”) comes into play. Divide that number by the first to determine the size, in millimeters, of the image that the binocular delivers to your eye. An exit pupil of 7.1 millimeters (found in a 7×50 binocular) is about the highest useful number – any bigger and it would exceed the size of your own pupil – and an exit pupil of 5 millimeters (found in 10×50, 8×40, and 7×35 binoculars) is very good.
Light transmission is also determined by the type of glass used for the lenses and the number of coatings that have been applied to those lenses – the more the better. The best and most expensive binoculars contain BaK-4 glass and are fully multicoated to reduce stray light and to increase contrast.
Dos and Don’ts
Many camera and birding supply stores carry quality birding optics, as do selected outdoor outfitters and a number of online vendors. The exhibit halls at birding festivals are great places to shop for optics. Before buying a binocular, read a review or two, ask other birdwatchers about their binoculars, and try out several models. Find an instrument that feels comfortable in your hands, works well with your eyeglasses (if you wear them), and is light enough for you to hold up for half a minute or more without your arms getting tired.
A birding binocular will cost between $150 and $1,500. Purchase the best you can afford, and be careful when buying binoculars less than $75 in price. The quality of the glass and the durability of such models can be quite low, and they are rarely water-resistant.
When birding in the field, situations often arise where greater magnification is needed than binoculars can provide. Enter the telescope, or spotting scope among birdwatchers. Spotting scopes come with magnifications above 20x, and some have zoom eyepieces that can boost the magnifying power up to 75x.
A seabird in flight over the ocean may appear as a speck to the unaided eye, but training a spotting scope on it will reveal the shape of the bird and other identifying marks.
Shorebirds, which often feed in large groups along mudflats and shorelines, can not be approached too closely for fear of chasing them off. A spotting scope becomes the essential tool for picking up the birds’ subtle differences.
Want to take it one step further? Learn all about the art of digiscoping (taking digital photographs through a spotting scope) in our article Making the Connection.
10 terms every binocular buyer should know
- Power: Magnification, compared to your naked eye. Power is the first number in the binocular equation. An 8×42 binocular magnifies what you can see eight times.
- Objective: This is the front lens (opposite the eyepiece). Its diameter, measured in millimeters, is the second number in the binocular equation. An 8×42 binocular has an objective 42 mm across. Large objectives produce a brighter image than small objectives if power is the same.
- Exit pupil: The cylinder of focused light coming out of the eyepiece. A wider exit pupil delivers a brighter image. To calculate exit pupil, divide the diameter of the objective by the power. For an 8×42 binocular, it’s 5.25 mm. For an 8×25 pair, it’s 3.125 mm.
- Field of view: Everything visible within the dark periphery seen when you look through a binocular. For comparison’s sake, the horizontal width of the field of view is usually measured in feet at a distance of 1,000 yards from the binoculars. When comparing fields of view, do so with equal-powered binoculars. The field of view through 10x binoculars will be smaller than lower-powered binoculars.
- Eye relief: The distance, measured in millimeters, your eyes can be from the eyepieces and still see the entire field of view. Higher-power binoculars generally have shorter eye relief, as do those employing Porro prisms.
- Close focus: The nearest focused point allowed by the binocular’s focusing mechanism.
- Prisms: Internal blocks of precisely cut glass that fold the light passing through a binocular, delivering a rightside-up image to your eyes. There are two types: Porro prisms and roof prisms. Objectives in Porro-prism binoculars are offset from the eyepieces. Objectives in roof-prism binoculars are in line with the eyepieces.
- Coatings: Chemicals applied to binocular lenses to reduce scattered light and increase contrast. “Fully coated” means that all glass surfaces have one layer of anti-reflective coating. “Multicoated” means that more than one layer has been applied to the glass, further reducing reflections. “Fully multicoated” means that each glass surface has more than one layer. “Phase coatings” are unique to roof-prism binoculars. They remedy interference created inside the prisms.
- Nitrogen-purging: A fog-proofing process. Air is vacuumed out of a binocular and replaced with nitrogen, a gas without moisture. The binocular is then sealed. Nitrogen-purging will not prevent outer surfaces from fogging, but interior surfaces will remain clear of condensation when the binocular is taken from cold and dry to warmer and humid.
- ED glass: Extra-low dispersion glass. Made with compounds or elements such as fluorite or lanthanum, ED glass bends light with far less chromatic aberration, resulting in crisper, brighter views than non-ED glass. Lens systems made with this glass are called apochromats. ED glass is expensive and often drives binocular prices into four figures.