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Choosing binoculars for bird watching!!

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/25/2004 at 11:04
Stephanie View Drop Down
Optics Goddess

Joined: February/13/2004
Location: Native Texan
Status: Offline
Points: 1502

Choosing Birding Optics

Good binoculars are clear, powerful, easy to hold and simple to focus. You also want binoculars that you can use for long periods of time without causing eye strain. Because individual needs vary between birders, there is no one binocular or type of binoculars that is best for everyone. Let’s take a more in-depth look at choosing the best binoculars for you.

What are all those numbers?!?

Binoculars have a set of numbers on them referring to their magnification power and the size of their objective lens. These numbers are expressed as a formula such as 7 X 35 or 7-15 X 42.

The first number refers to the magnification. If this number is hyphenated it means that the binoculars are capable of a range of magnifications. In the example used above the 7-15 means that the binocular is capable of zooming between 7 and 15 power. Binoculars over 10 power may be difficult to hold steady enough to see the image clearly. Often these binoculars have provisions for mounting on a tripod.

The second number represents the size of the objective lens in millimeters. The larger the objective lens the more light it allows into the binocular and the brighter and clearer the image will be. Unfortunately, as objective lenses get larger, the optics get heavier and more uncomfortable to hold.

The Exit Pupil

Another factor in choosing binoculars is determining the "exit pupil." Ideally binoculars exit pupil should be 4 mm or above for general daylight use, or 6-7 mm recommended for low light work. To determine the exit pupil of a pair of binoculars, divide the size of the objective lens by the magnification. In the nonzoom example above we would divide 35 by 7 and determine that these binoculars have an exit pupil of 5 mm, a good size for general work. (The exit pupil is the diameter of the image as it leaves the eyepiece lens. If it is as large or larger than your eye's pupil diameter then your eyes can take full advantage of the images they receive. If the exit pupil is smaller than your eye's pupil diameter the image may appear dark and less clear.)

Eye Relief

Another important consideration, especially for those of us who wear glasses is eye relief. Eye relief is the maximum distance in millimeters that your eyes can be away from the eyepieces and still see the whole picture. Normal binocular eye relief ranges from 9 to 13 mm. This distance works well for folks with good eyesight. Most glass wearers need eye relief over 13 mm. Binocular manufacturers try to provide this relief through the use of rubber eyecups that can be rolled down. Often this is not enough! Some binoculars are constructed with extended eye relief for glass wearers. Many manufacturers add the letter AB@ the description of binoculars with long eye relief.

How Optics Work

All optics consist of a series of glass lenses and a housing. 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/or glass prisms. These are used to magnify or bend light. Generally speaking, the longer the light path the more magnification that can be produced.

Telescope In telescopes this is done by increasing the length of the tube that holds the objective and eyepiece lenses. There are practical limits to this.
Porro Prism Binoculars In binoculars reflecting the light into a folded path using a pair of prisms increases the length of the light path. This allows more magnification in a shorter housing than that available using a telescope. This also explains the general shape of common porro prism binoculars.
Roof Prism Binoculars Roof prism binoculars use advanced prism designs to bend the light in a very compact way. The lens alignment must be very precise. This explains why good roof prism binoculars tend to be very expensive.

Size, weight, and cost are other factors to consider, but that's an individual choice and is hard to predict. Getting good magnification and clear, bright images are very important, however. The best way to test for what's comfortable for you is by trying different binoculars before you buy them, either at the store or preferably by borrowing them from a friend. A good, economical choice would be a well-made set of 7 X 35 porro prism binoculars. If your budget cooperates, consider higher-priced binoculars which tend to be more ruggedly made, have better optics and cause less eyestrain.  For those people who did not know...

Edited by Stephanie
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