Field of view is the extent or the range of your visual area. In terms of optics, it is widest dimension an object is visible through the eyepiece of a scope or binocular. Binoculars are the most common form of optics purchased for sporting activities. It is important to choose the right features in a binocular for your activity. Field of view is an important factor when making that decision.
Field of view, or FOV, is expressed in two ways. It is either expressed as the width in feet at 1,000 yards or in degrees of field. When the field of view is expressed in feet, it is called the linear field of view. When it is shown in degrees, it is referred to as angular. It is simple to convert one measurement to the other. The linear field of view is measured in feet at 1,000 yards. One angular degree is equal to 52.5 feet. Once the angular field of view, or degrees, is known, multiply that figure by 52.5. In most cases, the field of view is often indicated in degrees on the outside of the binoculars. For example, if the angular field of view is six degrees, then multiply 6 by 52.5, for a total of 315 feet at 1,000 yards, or a linear field of view of 315 feet. Similarly, if the linear field of view is known, divide that number by 52.5 to obtain the angular field.
The term real field of view refers to angle of the visual field which can be seen without moving the binoculars. It is measured from a central point on the objective lens. A higher value will translate to a wider visual field. The term apparent field of view refers to the angle of the magnified view. A large apparent field of view will provide a wide field of view even at a higher magnification. Optics with a wide field of view will make it easier to follow moving objects.
Binoculars display two numbers. The first number, followed by an X, is the magnification. It is the degree by which an object will appear larger. Binoculars are manufactured in a range of magnification, but lower magnification binoculars, such as 5x to 8x, are the most popular and typically offer a wider field of view. Higher magnifications, 10x and above, are available for long distance viewing, but generally have a narrower field of view.
The second two-digit number is the aperture. This is the diameter of the objective lens measured in millimeters. The size of the objective lens determines the light gathering capability of the binocular. The greater the light gathering capability, the brighter an object will appear. However, the size of the objective lens will affect the physical size of the binocular. A larger lens diameter will mean a larger and heavier binocular.
The field of view, in addition to the magnification and aperture size, are important considerations when looking to purchase an optic. Binoculars with a wide field of view are popular for bird watching, wildlife watching, or sporting events, as it is easier to follow objects that move quickly. A general purpose binocular, such as 7×35 or 8×42, would be a good choice for these activities. Lightweight enough to carry hiking, these binoculars have a wide FOV yet are powerful enough to distinguish detail. However, if you intend to bird watch or wildlife watch near wet, marshy areas or in wet weather conditions, you may want to be sure that the binoculars you purchase are waterproof and nitrogen or argon purged for fog proofing.
In choosing binoculars for astronomy, the aperture will be an important specification. For this activity, a binocular such as 7×50 or 10×50 is preferred. The 10x magnification will aid in viewing the planets, but a larger aperture will be important in dim and low light conditions. A larger aperture and greater light gathering ability will provide better resolution and allow you to study the planets in better detail. Binoculars with magnifications of 16×70 or 20×80 will benefit from being mounted on a tripod for steady viewing.
For boating activities, the marine binocular traditionally purchased is the 7×50. The 7x magnification allows a wide field of view and makes steady viewing easier with the movement of the water. The 50mm lens offers color and detail when viewing the horizon. When choosing a binocular for hunting, a lower magnification, such as 7x or 8x, provides a wide field of view to scan fields and tree lines. A 42 or 50mm objective lens would be a good choice for low light conditions, as a hunter might expect game to be more active at dawn or nightfall.
Optics will open you to a variety of scenery and wildlife, but understanding the field of view and other features will aid in purchasing an optic that is right for you.