Optical Field of View is the actual distance across your viewing field (left to right) when looking through a binocular or spotting scope. It can be measured in two different ways: linear feet at 1,000 yards (also known as True Field of View) and angular degrees (or Apparent Field of View). You can easily convert from one measurement to the other by knowing that one angular degree is equivalent to 52.5 feet. Thus, if you know the angular field of view in degrees, multiplying that number by 52.5 will give you the linear field of view of feet at 1,000 yards. Likewise, dividing the linear field of view by 52.5 will give you the angular measurement.
There are a few positive viewing points regarding a wide field of view: It’s easier to aim and stay on a moving subject, therefore binoculars with a wide field are generally preferred by birdwatchers. A typical 8x42 binocular has a field of view around 340 feet at 1,000 yards or 6.5 angular degrees. Binoculars with a field of view 390 feet at 1,000 yards or above are generally considered wide field.
A common misunderstanding is that the objective lens diameter (32mm, 42mm, 50mm, etc.) is the field of view. Even binoculars with a narrow objective lens diameter can have a wide field of view; it', because it has more to do with the physical length of the binocular and the configuration and design of eyepiece lens elements. Hence, the field of view cannot be determined by the two numbers of a binocular, e.g. 8x42, 10x50, etc.; you must look at the manufacturer’s provided specification for it.
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