Buying Optics for Bird Watching
What do you need to know?

Binoculars are an essential tool for visually identifying birds and observing bird behavior. Using quality optics makes watching birds and other wildlife more exciting and enjoyable. In addition to birding, bin-oculars are designed and used for various types of outdoor recreation including spectator sports, astron-omy, hunting, sailing and fishing at sea. Other nature-watching activities such as butterfly-watching and dragonfly-watching also are becoming popular, demanding good optics that birders demand but with close-focusing feature. With so many applications it is important to purchase a pair of binoculars specifi-cally suited for watching birds.

Along with a wide assortment of binocular brands and size configurations to choose from, there is a broad range in prices, extending over the $3,000 mark to under $100. With optics, cost variation reflects the optical and mechanical quality of each model. A pair of binoculars is priced according to key compo-nents such as the type of glass used for lenses. The quality of glass directly affects image clarity throughout the viewing area. High quality glass delivers a clear flat field across the entire field of view. Other key components include the type and coverage of anti-reflection coatings on the glass surfaces, the size and type of prisms, eye piece design and focusing mechanism. Other price factors include dura-bility features and weatherproofing.

For birding, a good pair of binoculars should:
  • Reveal sharply fine details of feathers and other features of a bird
  • Be bright enough to see a bird's distinguishing field marks in low light
  • Have a field of view wide enough to locate a bird and track birds in flight
  • Have an adequate depth of field to find and follow active birds
  • Accurately portray color
  • Provide quick and easy focusing
  • Work comfortably with or without eyeglasses
  • Be lightweight enough to tote and use all day
  • Feel comfortable in your hands

Key terms to consider before purchasing binoculars
Magnification: Binoculars have a combination of two numbers, for example 8x40. The first number indi-cates the power or magnification¾how many times closer an object will appear. Magnification can range from 3x as in opera glasses to 25x and beyond for naval and astronomical use. The most practical mag-nification for birding, however, includes magnification between 7x and 10x. One important factor to keep in mind is that as an object is magnified, movement is equally magnified, which makes it more difficult to hold higher power binoculars steady for a clear view. Therefore, 7x binoculars will be easier to hold steady than a pair of 10x binoculars. In addition, heat waves rising from fields, beaches or marshes also are magnified, which creates a distorted image with higher power binoculars and scopes.

Magnifications of 7x and 8x are popular for many birding situations. Higher power binoculars, such as 10x or 12x, are typically adequate for birding in open landscapes and on big water; places where birds are distant, such as a floating group of ducks or migrating hawks. But they can be hard to hold steady.

Some beginners, especially the very young and the very old, have used 6x binoculars for backyard bird-watching and nature study.

Objective Lens: The number following the magnification describes the diameter of the lens at the far end of the barrels which is measured in millimeters. This lens is called the objective lens. The objective lens captures available light, so, the larger the objective lens, the brighter the image.

Optical coatings: Another factor determining the amount of light transmission through the lens is the type and coverage of antireflection coatings on the lenses. The more air to glass surfaces that are coated, the less light will be lost to reflection. Multiple layers of coatings further increase the brightness. It is important to understand the labeling terms describing antireflection coatings. Coated refers to a single layer on some lens surfaces, usually the visible lenses at both ends. Fully Coated means all air to glass surfaces are coated with a single layer. Multi-coated means some lenses will be multi-coated and some will only have a single coating. Fully multi-coated describes the brightest and highest quality in which all air to glass surfaces are coated with multiple layers of antireflection chemical coatings.

Field of view – When you look through binoculars, the width of the area within sight is the field of view, measured in feet at a distance of 1000 yards (or meters at a thousand meters) or measured in degrees as in Angle of View. A wide field of view is important when tracking birds in flight and critical when trying to locate a bird within a vast scene. Fast-moving songbirds are easier to find with a wider field of view. The field of view decreases as magnification increases, so a 7x magnification will tend to have a wider field of view than a 10x binocular. Eyepiece design also determines the field of view.

Depth of view – As you view the magnified image, this is the distance from the viewer to infinity in which the image is clear, or in focus. This is a difficult to describe or quantify, but it is an important characteristic of any optical device used to view active wildlife, including small, lively birds. Lower power binoculars tend to have greater depth of field than higher powered optics. This is one of the characteristics of excellent quality birding binoculars that make bird observation easier. It is a particularly important characteristic to choose for optics when they're used where there is a lot of vegetation that makes following a bird challenging. Depth of view is often an overlooked, but important characteristic of a good bird-ing optic. It is easier to appreciate in the field than in a store with mostly flat surfaces. More expensive optics tend to have a better depth of view.

Focus Mechanism – Birds fly, dive, swim, walk and run, but rarely remain motionless for very long. The ability to quickly focus your binoculars on a bird is crucial, which makes ease of focus an important fea-ture to consider. Most binoculars have a single knob or wheel at the hinge in the center ¾ center focus binoculars. This knob should be easy to control with your fingertips. The complete range of focus should not rotate more than one and a half turns and one turn (360 degrees) or less is best. Some newer models are equipped with a variable speed focusing gear, which provides slower focus at close range for optimum focusing precision and fast focusing at farther distances to help you hit the mark.

Eye relief – Eye relief is the distance your eye can be from the eyepiece and still see the entire field of view. With eyeglasses on, your eye is farther away from the ocular lens, which requires longer eye relief thus reducing the field of view. Binoculars for birding are designed with a range of eye relief from 15mm to 20mm. Adjustable eye cups make viewing comfortable with or without glasses. It is important for glass-using buyers to try out new binoculars with eyeglasses on.

Close focus – The minimum focus of binoculars is something to consider. Close focus is important for observing details when birds are near, such as at a feeder just outside the window. It also allows a good look at a bird on a nest that you do not want to disturb by approaching too closely. This also is an important characteristic of binoculars that will be used for observing butterflies, dragonflies, reptiles, amphibians, flowers, or other subjects. With some newer models, you can focus on objects at your feet.

Exit Pupil – Pull your eyes away from the eye cups, the small circle of light you see in the eyepiece is the exit pupil. As lighting fades, our pupils dilate to maximize available light, on average 4 to 5mm. Your eye's ability to see in low light will exceed a small exit pupil. So, the exit pupil of birding binoculars should be a minimum of 4mm. The diameter of the objective lens divided by the magnification equals the exit pupil (i.e., a pair of 8x32 binoculars would have an exit pupil of 4mm, because 32 divided by 8 equals 4).

Size and shape of binoculars – The objective lens (the lens at the far end of the binoculars) produces an upside down image, backwards, too. Prisms between the objective and ocular or eyepiece lens invert the image so that you see the object right-side up and correct right to left. Binoculars come in two general shapes depending on the shape and arrangement of the prisms.

Roof prisms are arranged in line with the lenses resulting in slim, barrel-like optical tubes. These are the "H" shaped binoculars that often cost more than porro prism binoculars. This alignment allows the focusing mechanism to fit inside the housing which contributes to the sleek shape and comparatively lighter weight. The roof prism design is usually more weatherproof as well. The sharpest and brightest roof prism binoculars are phase corrected and require precise alignment of the prisms. This technical precision for high quality roof prism models increases the price compared to porro prism models of the same class.

Porro prisms are larger and aligned side by side. These are the "W" or "M" shaped binoculars that many birders use before they can afford to buy roof prisms. With porro prism binoculars the ocular lenses and objective lens are offset creating a broader housing and an overall heavier piece of equipment. The advantage to porro prisms may be a wider field of view, a greater depth perception and less expense as compared with roof prism binoculars of equal quality. This is where there are some real bargains in binocular-buying.

Full size binoculars offer objective lens sizes of 40 and 42mm, and larger. They are high performance optics with maximum brightness and field of view. The drawback is the larger size and weight, some-times as much as 10 to 20 ounces more than mid-size and compact models.

The objective lens in mid-size binoculars range unofficially from 30 to 32mm. The compromise for these lighter and less bulky binoculars is somewhat less detail in low light. However, some of the very expensive binoculars compensate for this limitation with light-enhancing lens coatings. These are becoming some of the most popular models among advanced birders because of their portability and low weight.

Compact binoculars are very light, portable and convenient to pack or carry. The objective lenses are small, some less than 30mm, which limits image clarity and detail, especially in low light and at distances more than 100 feet. Compacts can be more difficult to locate an object and to focus on it. With a smaller exit pupil, compacts require more precise alignment of your eye with the eye piece to view the entire field of view.

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