Binoculars for Bird Watching

The second most important tool for bird watching after a field guide is a pair of binoculars. Binoculars for bird watching are essential for distinguishing details not seen by the naked eye. Your binoculars should feel comfortable in your hands and provide clarity with easy focus. Weight and weather-proofing are other things to consider when choosing a pair of binoculars. There is a wide selection of binoculars available ranging in price from $30 to hundreds of dollars. Binoculars generally come in two size formats, compact and full-size models. Choosing the right pair depends on how you intend to use them. For backyard bird watching you should be satisfied with a pair in the lower price range. If you plan to use your binoculars in the field, they should be sturdy, lightweight and be weather-proof.

How Optics Work
Inside the binoculars is a series of glass lenses. The lens nearest the eye is known as the eyepiece. The lens farthest from the eye is known as the objective lens. Inside the housing between the eyepiece and the objective lens are combinations of other lenses and glass prisms. These are used to magnify or bend light. A longer light path gives more magnification.

The following information will help you decide which binoculars you should buy.

Magnification:  The most common magnification sizes used by birders are between 7X and 10X. 10X is generally regarded as the largest magnification which is practical for hand held binoculars. 7X may seem to a bit weak, but is easy to use, and combined with a large objective lens, provides good performance in low light conditions. 8X or 9X is most popular among birders, although many birders seem to like the power of 10X.

Size of Objective Lenses. Objective or light gathering lenses are important as they determine the amount of light that is received by the binoculars. 7X32 means that the objective lens is 32mm in diameter.  The amount of light the binocular receives is a function of the area of the lens rather than diameter. This means that 7X50 objectives gathers more than twice as much light as 7X35 lenses. The size of the objective lens affects both image resolution and performance in poor light conditions.  All things being equal a 10X50 will provide better resolution than a 10 X40.

Field of View.   The wider the field of view, the easier it is, to scan the terrain, spot up a bird, or hold a moving bird in your field of vision.

Depth of Field. Depth of field describes how much of the field is in focus at any one time. Binoculars with a poor depth of field require a great deal of attention on focusing in order to keep things in sharp focus. It is also more difficult to spot a bird which has not yet been focused on if the depth of field is poor. Binoculars with a good depth of field makes birding more pleasurable and concerns you less with focusing and more with identifying the individual characteristics of the bird in view.

Eye Relief. Eye relief describes the minimum distance between the exit lens and your eye, this is important if you wear eye glasses. A long eye relief means that you can hold the binoculars further from the eye while wearing glasses and still get the full benefit of the binoculars.  Good eye relief is 16mm upwards.

Size of Exit Pupil. The exit pupil size is the size of the pupil through which the light reaches your eye. In good light a 4mm exit pupil produces the same brightness as a larger exit pupil, This however changes in dim light, where the larger exit pupil provides a real advantage. A less obvious advantage of a slightly larger exit pupil is the ease of use. A small exit pupil requires precise alignment with your eye, and tends to be more strenuous to use, particularly if you are birding for long periods. A larger exit pupil is more relaxing to use.

Close Focus. The capacity to focus on those birds that come within four or five meters is an essential factor for good birding binoculars. Obviously the closer the better, but a close focus of 10 feet is desirable if you are short-sighted. 

Weather-Proofing. Weather proofing varies from full weather proofing to shower proofing to no weather proofing at all.  Fully nitrogen-purged and sealed binoculars are ideal, but come with at higher price.  For backyard birding, full weather proofing is not generally necessary and many of the better quality mid-range binoculars provide adequate shower proofing. Care must be taken in rainy conditions and binoculars may have to be nursed in very wet conditions.  If you are going to be birding in rainy conditions make sure you get a pair with a rain guard for the front lenses.

Type of Lenses and Lens Coatings. Special coatings on lenses found in binoculars are aimed at reducing reflection, and it is here that the greatest advances in image quality have been made.  Cheaper binoculars usually have coatings only on the external lenses, while better binoculars have coatings on all lenses. Single coating lenses reduce light loss to 1.5 % from about 4% for uncoated lenses. The best multi-coatings achieve a figure of less than 0.5% per lens. Considering that a pair of binoculars typically has more than 10 lens surfaces the difference between fully multi-coated and coated can be quite substantial.

 

Key decisions for making a purchase: Magnification, Style and Price

1. Magnification: choosing between 8X or 10X

This is the most common question that birding binocular buyers ask.  Obviously one should get a better, more magnified view with a 10 power binoculars. In reality, the larger magnification is more difficult to handle, and it is doubtful whether even experienced birders actually see more through the more powerful pair. 

The key issue is quality and the diameter of the objective lenses. All things being equal, a 10X50 will provide better image sharpness then a 10X40, as it gathers more light. Good quality 8X32 binoculars will provide a better, more clear image than most 10X binoculars will,  and benefit from the ease of use and the stability of the image you are seeing.

8X32 and 10X40 have about the same magnification to objective lens ratio and are generally regarded as the standard, or minimum for birding, where one generally needs good light gathering ability. 10X50 or 8X42 will provide a better image and are popular among birders willing to carry the extra weight. In addition these binoculars may also offer a wider field of view, which some birders regard as important, as it allows one to pick a bird up more easily.

2. Style: Porro vs Roof Prism?

Weight advantage, compact and durable design and the elegant shape of roof prisms has contributed to making them almost universal among top birders. In fact the roof prism design is not as good optically as a porro prism design of the same specifications. This means that you can get the same quality view at a lower price by selecting binoculars with porro prisms. Since many manufacturers have produced their best binoculars in porro prism format, there is now a move back to these binoculars, although there is a compromise in weight and bulk and invariably build quality as well.

3. Price: How much should you spend?

Simply put, buy what you can afford. At one time, there was a big difference between the top-of-the-line binoculars and the average binoculars. Recently, binocular manufacturers have become more aware of the needs of birders and far more bird-worthy binoculars exist. Full multi-coating is now common on binoculars in the mid price range, as is the combination between large objective lenses, good depth of field and a relatively wide angle. This means that there is not all that much to separate a good mid range birding binocular from the top of the line models. What usually does separate the pairs is a marginal optical difference, water-proofing and rugged quality, as well as the more compact roof prism design.

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