Binoculars come in two basic designs; Porro prism and roof prism.
Both of these binocular types are available either full-sized or compact size.
The majority of very serious birders use top-of-the-line roof prism binoculars. It’s easy to understand why when you get an opportunity to compare roof prism with most Porro prism glasses. You’ll literally see the difference.
The name of the binocular is usually followed by two numbers on either side of an "X". The first number is the power or magnification of the lenses. The second number tells you the size of the objective lens (the front lens).
Typically, birders prefer binoculars with a 7X to 10X magnification. Most of the lower magnification binoculars allow for better viewing at close range and are easier to hold without noticeable hand shake vibration. (Hand shake can be a problem for some people when using greater magnification.)
The size of the objective lens affects the size of the binoculars. This lens will also determine the amount of light transmitted by your binoculars.
Usually, the more light transmitted, the better you’ll see what you’re viewing.
This is where the second number comes in. The second number divided by the first number determines the size (in millimeters) of the image the binoculars will deliver to your eye. This is called the exit pupil length. Any number between 4 and 6 is considered very good.
Full-sized binoculars (such as 8x40, 8X42, or 10X50) have larger objective lenses. They’re desirable for birding because they have the greatest light-gathering ability and best overall optical performance.
Compact binoculars (such as 8X20, 8x25, or 10X25) are sometimes desirable for birding because they’re more portable than full-sized models. Compact models tend to work better during daylight hours. They can loose color and resolution at dawn, dusk, or in deep shadows.
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