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Spotting Scope - Part 2 |
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More on Light Gathering Ability At dusk, the average person's pupil dilates to 5.5 millimeters, so optics used for early morning or late afternoon wildlife viewing should have at least a 5mm exit pupil. To determine the exit pupil size, divide the size of the front lens by the magnification. For 7 x 35 binoculars, for example, the exit pupil is 5mm minus 35 divided by 7 equals 5. Increasing the size of the objective lens to capture more light often forces the optics designer to also increase the size and weight of the instrument. In daylight you can get by with as little as a 2mm exit pupil, and that makes light-weight mini-binoculars and scopes an option. It is possible to buy binoculars with an exit pupil larger than 5mm. However, since the human eye dilates to only about 7mm in total darkness, these binoculars may gather more light than the average person really needs. When optics makers push up the exit pupil size, they also must increase the price, since they use more ground glass for the front lens.
Field
of view Manufacturers list the field of view in feet, giving the width of scene a binocular will show 1,000 feet away. Most birders prefer wide-angle glasses, since they allow you to take in more of the landscape and keep track of passing species. Others want to be able to observe as much detail as possible. Generally, the higher the magnification, the narrower the field of view. The final technical considerations in making aselection are in Part 3. Return to Part 1 Return to Binoculars & Scopes Homepage Focusing
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