Guide to Binoculars

Guide to Binoculars With so many binoculars to choose from, how can you pick the one that is best for your needs? This guide will help you to understand the differences between prism types, compact versus standard size binoculars, and what makes one binocular brighter than another. If you’re not familiar with binoculars, all the strange numbers and terms may make it difficult to choose the right one for your activities. Once explained, the terms and numbers are easy to understand, and they will help you sort through the wide selection that is available. TYPE OF PRISM All binoculars have glass prisms in each barrel so the image will be erect and not upside-down. Better quality binoculars use a high-density glass called BAK4 that provides a sharper image than less dense prisms. Binoculars have either porro prism or roof prism designs. If the binocular has a “jog” in each barrel, it is a porro prism design. The advantages of porro prism binoculars are cost and the greater three-dimensional viewing provided by the offset objective lenses. Roof prism binoculars are more compact, but are usually more expensive because the roof prism design requires very tight manufacturing tolerances. ROOF PRISM PORRO PRISM All binoculars contain prisms to make the images right side up and correct left-to-right. Binoculars with porro prisms are more common, since this style serves the purpose well and is less expensive. Roof prism binoculars are more compact, but require strict manufacturing tolerances and are usually more expensive. DEFINITION OF THE NUMBERS Binoculars are described with numbers, such as 7x35, 8x42, 12x50, etc. The first number is the magnification. A 7x35 binocular will make an object that is 70 feet away appear 7x closer, or 10 feet. If the point of using binoculars is to bring things in closer, isn’t the most powerful binocular the best? No, not necessarily. Binoculars magnify your movement along with object size, so high-powered binoculars are harder to keep steady than 7x or 8x models. Also, size and weight increases with magnifying power, so a 12x or 16x binocular will be more tiresome to hold. Most binoculars above 10x will accept an adapter for mounting on conventional photo tripods. Several manufacturers make binoculars with image stabilization that electronically reduces movement for steadier viewing. This type of binocular has sophisticated prisms that automatically adjust to counter the effect of movement, helpful when viewing from a moving vehicle or with high-magnification binoculars. Most binoculars have fixed magnification, but there are some zoom binoculars on the market that allow you to change magnification by moving a lever by the eyepiece. Zoom binoculars are more expensive than fixed magnification models and have a much narrower field of view, but they provide the viewer with variable magnification, a handy feature. The number after the “x” is the diameter of the objective lenses, in millimeters. The objective lenses are on the side opposite the eyepiece lenses. The objective diameter dictates how much light will enter the binocular and how bright the view will be. A 7x50 binocular is brighter than a 7x35, but also larger and heavier. Since the brightness is a proportion of magnification and objective diameter, the objective size needs to increase to keep a high-powered binocular as bright as a more moderate one. For example, 7x35 and 10x50 binoculars have the same brightness index, even though the 10x binocular has bigger objective lenses. The field of view is often marked on binoculars. It’s expressed as a measurement of feet at 1000 yards. The great the number of feet, the wider the angle of coverage. As the magnifying power increases, the field of view decreases. The eyepiece design also affects the angle of Field of View/Angle of View coverage. Objective lens Field of View Angle of View Exit Pupil Diameter Eyepiece 1000 yards Another number given in binocular specifications is the exit pupil diameter. This is the diameter (in millimeters) of the beam of light that exits the binocular eyepiece. The larger the exit pupil the brighter the binocular. The exit pupil diameter is the objective diameter divided by the magnification. For example an 8x40 binocular has an exit pupil diameter of 5. If most of your viewing will be in moderate to bright lighting conditions, an exit pupil diameter between 2 and 5 will be quite satisfactory. For viewing in dim light or at night, an exit pupil diameter above 5 will provide a much brighter image. Binocular specifications use another term to indicate brightness, called the relative brightness index. It is simply the square of the exit pupil diameter. A binocular with an exit pupil diameter of 3 has an RBI of 9, one with an exit pupil diameter of 5 has an RBI of 25. Binocular eyepieces consist of between three and six lens elements. Eyepieces magnify the image after it has passed through the prisms. Wide-angle binoculars have wide-field eyepieces that shorten eye relief. (The distance from the eyepiece to the eye.) This reduced eye relief may make it difficult for someone who wears eye glasses to see the entire field of view. Binoculars are usually supplied with rubber eyecups on the eyepieces. Eyecups block out unwanted peripheral light and can be folded flat against the eyepieces if necessary. Virtually all binoculars are water-resistant, but water can enter the optical system if they are dropped into water or are left unprotected in wet weather. Some binoculars are waterproof and have O-ring seals to keep out water. True waterproof binoculars are also filled with nitrogen to keep out air and moisture, and to prevent internal fogging. Waterproof binoculars are recommended for those who will use them around water in in all types of weather, such as boaters and hunters. FOCUSING Most binoculars have two focusing adjustments. A center focusing knob moves the eyepieces (either internally or externally) to focus the binoculars over the range of object distance. The second focusing adjustment is the diopter adjustment found on at least one eyepiece. There’s usually a difference in vision between the right and left eyes, so the diopter can be adjusted to bring your vision into balance. Once you’ve adjusted the eyepiece(s) to your vision, you won’t need to readjust them. HOW TO CHOOSE BINOCULARS Each binocular manufacturer produces many different models, because no one binocular is right for everyone. There are five major factors to consider when selecting binoculars - magnification, brightness, size/weight, field of view, and cost/value. A serious bird watcher who is out at dawn or dusk frequently will want binoculars that are extra bright. A sports fan who wants binoculars for watching daytime baseball games won’t need the brightness, but the angle of view will be important. For general use, binoculars that have 7, 8 or 9x magnification are best. They provide ample magnification without as much movement and vibration as hand-held high-powered binoculars. Some activities such as surveillance, astronomy and bird watching may benefit from the greater magnification provided by 10 or 12x binoculars. Size and weight is important to some people, such as hikers and hunters, and for them, compact binoculars will be ideal. COMPARING THE DIFFERENT TYPES OF BINOCULARS Both porro prism and roof prism binoculars are available in compact models. They typically have 7, 8 or 9x magnification and objective diameters between 21 and 25mm. Compacts are indeed small, and weigh 12 ounces or less. They are often used more frequently than bigger binoculars, because their small size makes a compact type more appealing to take with you on a walk through a park, for example. The main drawback of compact binoculars is their lack of brightness, compared to standard size binoculars. Standard size roof prism binoculars are more compact than standard size porro prism binoculars and the stream lined design reduces their bulk, too. Roof prism binoculars are more expensive, due to the strict tolerances required by this prism design. Standard size porro prism binoculars provide a good blend of magnification, brightness, field of view and cost. Plus, the offset objective lenses increase the three dimensional viewing effect. If you can’t decide what type of binocular to buy, you can’t go wrong with a 7x35 porro prism binocular. 3. Typical Specifications for 8X Binoculars 1. 4. Binocular Magnification Style Compact Porro 8X Prism 1 Compact Roof Prism 2 8X Field of Exit Relative DimenPupil Objective View @ Diameter 1000 yds. Diameter Brightness sions 24mm 393 ft. 3.0mm 9.0 Weight 4.2x4.2" 9.9 oz. 25mm 430 ft. 3.1mm 9.7 4.1x4.7" 12.4 oz. 2. All four binoculars in this group have 8x magnification, but they vary in size, brightness, field of view and price. Standard Porro Prism 3 Standard Roof Prism 4 8X 40mm 500 ft. 5.0mm 25.0 5.4x6.7" 31.1 oz. 24 oz. 8X 42mm 324 ft. 5.3mm 27.5 6.1x5.1" HOW TO TAKE CARE OF BINOCULARS Binoculars are precision optical instruments, but they require very little care and maintainance. Their main enemy is rough handling. Slight bumps won’t disturb the alignment of the prisms and lens elements, but a serious drop will. Always use a neck strap to prevent this from happening. The objective and eyepiece lenses will pick up dust and lint. The best way to remove it is with canned air or a lens cleaning brush. As a last resort, use photographic quality lens cleaner and tissue to remove fingerprints and smudges. Binocular lenses have special chemical coatings like camera lenses, and can be damaged by eyeglass cleaners. Whenever you store the binoculars, put the lens caps back on to help keep the lens surfaces cleans. Courtesy of— Porter’s Camera Store ph. 1-800-553-2001 www.porters.com