How To Choose Binoculars
How to Choose Binoculars
Most binoculars are multi-purpose and may be suitable for a variety of different activities including birdwatching, nature observation, hunting, sporting events, hiking, concerts and more. A high quality binocular will allow you to see superb details, identify birds from great distances, and is the most important tool for any birder. The binocular you choose depends on what you're going to be using them for and under what conditions they will be used. Binoculars come in a variety of shapes, sizes, and specifications. No one binocular is perfect for all situations. Determining their primary use is one of the most important factors in choosing the right binocular for your needs. Essentially, a binocular is two low powered telescopes mounted together. Binoculars allow a more comfortable viewing experience by using both eyes instead of one and they have two basic functions; to gather more light than the unaided human eye and to enlarge the image you're looking at. You'll want to consider some of the following factors when choosing a binocular; magnification (power), field of view, size, weight, eye relief, exit pupil, prisms, type of coatings, aperture, and near focus.
The power (or magnification) of a binocular is represented by the first number for hand held binoculars and indicates how much larger, or closer, the object will appear than with normal unaided vision. For example, with a 8x42 binocular, the first number (8) is the binocular power. A binocular of 8 enlarges (or magnifies) the image eight times the size as seen by the normal, unaided eye. More power is not necessarily better. As magnification increases, brightness and clarity may decrease and the field of view is usually more restricted. In general, the lower the power the brighter the image will appear. However, other factors also affect brightness such as objective size, exit pupil, type of coatings and the type of prisms in the binocular. If you plan to use your binocular for close range viewing, such as in your backyard or woodland areas then a 7 or 8 power binocular may be appropriate. This magnification range usually provides a larger field of view, which is important when viewing objects at close distances. Binoculars in this range of magnification usually perform well under low light conditions such as in the early morning, early evening hours or when shadows are a factor. This same type of binocular usually works well for viewing fast-moving sporting events because the large field of view allows the action to remain in the viewing area. Binoculars with a higher magnification of 8, 9, or 10 are usually appropriate for long distance viewing or where greater detail is required. For instance, a critical field mark identification is needed or in the case of an object that is difficult to approach, such as wildlife.
Objective lenses are the front lenses of the binocular and they are measured in millimeters. For example with a 8x40 binocular, the second number (40) is the diameter of the objective lenses. A larger objective lens size (sometimes called aperture) will gather more light allowing for greater detail and clarity. This is especially true under low light conditions such as dawn, dusk, or overcast days, etc. Once light has been gathered by the objective lenses into the binocular, other factors determine how much light is transmitted through the system, and many of these factors determine the brightness and clarity of the image you see. These factors include magnification, exit pupil, eye pupil size, the presence and type of anti-reflection coatings used, and the size and quality of the optical glass and prisms used in the construction of the binocular.
Erecting prisms are used in binoculars to correct an inverted image. There are two types of prism systems: Porro prism (offset barrel) and Roof prisms (straight barrel). The Porro design is capable of delivering a wide field of view with excellent image sharpness. Porro prisms come in two common styles BK-7 and BAK-4. The difference between the two is the glass density. BK-7 utilizes boro-silicate glass and BAK-4 uses barium crown glass. The BAK-4 is the finer glass and eliminates internal light scattering and produces a sharper image. Binoculars with BAK-4 prism are usually more expensive than binoculars using BK-7. Porro prism binoculars tend to be bulkier, heavier, and larger than those with Roof prisms. Roof prisms are usually lighter in weight and more compact. They are more complex and more difficult to manufacture than Porro prisms so they typically cost more. Roof prisms tend to stand up to rough treatment better than Porro prisms because of their compact design. Roof prisms split the light entering the barrels into two paths. Light reflected from one roof surface is 1/2 a wavelength shifted from the light hitting the other roof surface. This is sometimes referred to as "out of phase" or "phase shift". Even though the light waves are forced back together by the time they reach the viewer's eye, this process does reduce contrast and image resolution. This effect does not occur in Porro prism designs. Some manufacturers coat the roof with an anti-phase shifting material. This improves image quality, contrast, and a sharper view. A phase corrected roof prism binocular is equal to (not better or worse) a Porro prism binocular in optical performance assuming all else is equal.
Field of View
The field of view is the size of the area you can see with a binocular. This is most commonly measured in linear feet at 1000 yards or may be expressed in angular degrees. The greater the field of view, the greater the area you will see in the image. Most binoculars have the angular and/or linear field of view indicated on the binocular. However, if you only know the angular field, but want to know the linear field, multiply the angular field by 52.5. This formula can be used to find the linear field of any binocular regardless of the manufacturer. For example, if the angular field is 8° then multiply 8x52.5=420 feet. 420 feet is the field of view at 1000 yards. Typically, the greater the power, the smaller the field of view. However, other factors such as the type of eyepieces used, and the focal length of the objectives determines the actual field of view. The size (diameter) of the objective lens (the front lenses) has absolutely no bearing on the field of view.
Size & Weight
Binocular sizes vary considerably, from pocketsize, to compacts, to standard-size, and even tripod-mountable binoculars. Weights can also vary significantly with typical standard-size models ranging from 20 to 30 ounces. Construction materials also play a part in the weight of the binocular. Materials such as polycarbonate and die cast aluminum make a binocular rugged and lightweight. Pocketsize and compact binoculars may be considered when size and lightweight are the most important considerations. Standard-size binoculars usually deliver the required exit pupil in low light conditions and should be considered when size and weight are not the most important considerations.
Eye relief is the distance, measured in millimeters, that a binocular can be held from the eye where you can observe the full field of view comfortably. This is especially important for eyeglass or sunglass wearers. Eyeglass or sunglass wearers need a longer eye relief, but they also suffer from peripheral light loss. It is best, if possible, to use a binocular without glasses. If you must wear glasses, be sure to purchase a binocular with fold down rubber eyecups (available on many models), to obtain the widest field of view. Longer eye relief is beneficial even if you do not wear glasses. It increases your comfort because you do not have to press hard against the eyecups to see the entire field of view. Eyeglass wearers typically need a minimum of 16-20mm of eye relief to enjoy full field of view.
The exit pupil is the diameter of the beam of light; measured in millimeters that leaves the eyepiece of the binocular. You can see the exit pupil, as a circular beam of light, by holding your binoculars out at arms length and looking through the eyepiece at a light source. In general, the larger the exit pupil, the brighter the image. Large exit pupils are advantageous when viewing in low light conditions, such as dawn or dusk or when observation is going to be done under conditions that will not allow the observer to keep the binocular steady. If the exit pupil of a binocular is larger than the entrance pupil of the eye then some light will not be seen by the observer. The image will continue to have the same brightness regardless of how much the exit pupil exceeds the entrance pupil of the eye. If the exit pupil of a binocular is smaller than the pupil of the eye however, then the image will seem dim to the observer. Typically, the eye pupil changes in size from 2mm up to 9mm, depending on available light. Pupil size varies greatly, but for most of us, the size of our pupils decreases as we grow older. To calculate the exit pupil of a binocular, divide the objective by the power. For example 8x40 binoculars is 40 divided by 8 = 5mm.
Optical Coating Types
Coatings reduce light loss and glare and increase light transmission and contrast. Increasing the amount of light that actually reaches your eye improves brightness and overall optical performance. Coatings are applied to objective lenses, oculars, and prisms. There are four types of coatings. Below are definitions of each.
Coated Optics: One or more surfaces of one or more lenses has anti-reflective coating.
Fully Coated: All air-to-glass surfaces have been coated. If any plastic lenses are used they are most likely not coated.
Multi-Coated: One or more surfaces of one or more lenses have been coated with multiple films. Some surfaces could be single coated or some not coated at all.
Fully Multi-Coated: All air-to-glass surfaces have multiple films. If you can afford it, buy a binocular with fully multi-coated lenses; light transmission will be higher and the image will be brighter.
Near focus is the closest distance to the observed object that the binocular can be used while retaining a sharp focus. The most important step in selecting a binocular is to first determine what their primary use will be and under what conditions they will be used. Building on this first step, will help you determine what features and considerations are most important. No one binocular is perfect for all situations, but careful consideration of all the factors discussed will allow you to select a binocular right for you and your needs.