Binocular Briefs
inoculars are two identical or mirror-symmetrical “telescopes” that are mounted side by side and aligned to point accurately in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. It is customary to categorize binoculars by the magnification x the objective diameter in mm; e.g. 7x50.
Unlike a monocular telescope, binoculars give users a three dimensional image: the two views , presented from slightly difernet viewpoints to each of the viewers eyes, produce a merged view with depth perception. There is no need to close one eye to avoid confusion as is usual with monocular style telescopes (spotting and rifle scopes). As you will soon see the physics of optics dictates some give and take in the desired features of any particular style.
CONTENTS
1.0 OPTICAL PARAMETERS
- Magnification
- Objective Diameter
- Field Of View
- Exit Pupil
- Eye Relief
2.0 OPTICAL TRIANGLE LAWS
- Increase Magnification
- Increase Eye Relief
- Increase Eye Relief and Field of View
3.0 OPTICAL DESIGN
- Galilean
- Porro Prism
- Roof Prism
- Porro vs Roof Prism
- Prism Glass
4.0 OPTICAL COATINGS
5.0 MECHANICAL DESIGN
- Focusing and Adjustment
- Image Stabilization
- Alignment
6.0 FIRST TIMER BINOCULAR SELECTION RECOMMENDATION
- Magnification
- Field of View
- Eye Relief
- Waterproof
7.0 PROPER BINOCULAR USAGE
- Adjust Eyecup
- Adjust Interpupillary Distance
- Adjust focus and then make diopter adjustments if needed.
- Find the birds!
OPTICAL PARAMETERS
Binoculars are usually designed for the specific application for which they are intended. Those different designs create certain optical parameters. The parameters include:
Magnification
The ratio of the focal length of the eyepiece divided into the focal length of the objective gives the linear magnifying power of binoculars (sometimes expressed as "diameters"). A magnification of factor 7, for example, produces an image as if one were 7 times closer to the object. The amount of magnification depends upon the application the binoculars are designed for. Hand-held binoculars have lower magnifications so they will be less susceptible to shaking. A larger magnification leads to a smaller field of view and also means that more movement will be apparent in the view.
Objective Diameter
The diameter of the objective lens determines how much light can be gathered to form an image. It is usually expressed in millimeters. A larger objective diameter provides more light which translates into improved color resolution and detail; however as objective diameter increases so does the weight of the binoculars.
Field of View
The field of view of binoculars is determined by its optical design. It is usually notated in a linear value, such as how many feet (meters) in width will be seen at 1,000 yards (or 1,000 m), or in an angular value of how many degrees can be viewed.
Exit Pupil
Binoculars concentrate the light gathered by the objective into a beam, the exit pupil, whose diameter is the objective diameter divided by the magnifying power. The exit pupil can be viewed by holding the binoculars several inches away from your face. The round white "dot" that you see on the lens is the exit pupil. For maximum effective light-gathering and brightest image, the exit pupil should equal the diameter of the fully dilated iris of the human eye—about 7 mm, reducing with age. Light gathered by a larger exit pupil is wasted. For daytime use an exit pupil of 3 mm—matching the eye's contracted pupil—is sufficient. However, a larger exit pupil makes alignment of the eye easier and avoids dark vignetting intruding from the edges. Larger exit pupil provides better dusk/dawn viewing. The less light that you will be working with the greater the importance of exit pupil size on your choice of binoculars.
Eye Relief
Eye relief is the distance from the rear eyepiece lens to where the image is formed. It determines the distance the observer must position his or her eye behind the eyepiece in order to see an unvignetted image. The longer the focal length of the eyepiece, the greater the eye relief. Binoculars may have eye relief ranging from few millimeters to 2.5 centimeters or more. Eye relief can be particularly important for eyeglass wearers. The eye of an eyeglass wearer is typically further from the eye piece which necessitates a longer eye relief in order to still see the entire field of view. Binoculars with short eye relief can also be hard to use in instances where it is difficult to hold them steady. When eye relief is equal to or greater than 15mm, the binoculars are typically eye glasses compatable. Return to Top
OPTICAL TRIANGLE LAWS
Increased Magnification: If Magnification is increased- Eye Relief and Field of View will be reduced.
Increase Eye Relief: If Eye Relief is increased- Field of View will be reduced and vice versa.
Increase Eye Relief & Field of View: If both Eye Relief and Field of View are increased- Magnification will be reduced.
Note: These laws are most easily demonstrated while using a pair of zoom binoculars. Return to Top
OPTICAL DESIGN
Galilean Binoculars
Almost from the invention of the telescope in the 17th century the advantages of mounting two of them side by side for binocular vision seems to have been explored. Most early binoculars used Galilean optics; that is they used a convex objective and a concave eyepiece lens. The Galilean design has the advantage of presenting an erect image but has a narrow field of view and is not capable of very high magnification. This type of construction is still used in very cheap models and in "opera glasses" or theater glasses.
Porro Prism Binoculars
Double Porro prism design
Named after Italian optician Ignazio Porro who patented this image erecting system in 1854 and later refined by makers like Carl Zeiss in the 1890s, binoculars of this type use a Porro Prism in a double prism Z-shaped configuration to erect the image. This feature results in binoculars that are wide, with objective lenses that are well separated but offset from the eyepieces. Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal length of the objective and wider spacing of the objectives gives better sensation of depth.
A typical Porro prism binocular design
Roof Prism Binoculars
Abbe-Koenig "roof prism" design
Binoculars using Roof Prisms may have appeared as early as the 1880s in a design by Achille Victor Emile Daubresse. Most roof prism binoculars use either the Abbe-Koenig prism (named after Ernst Karl Abbe and Albert Koenig and patented by Carl Zeiss in 1905) or Schmidt-Pechan prism (invented in 1899) designs to erect the image and fold the optical path. They have objective lenses that are approximately in line with the eyepieces.
Porro vs. Roof Prisms
Roof-prisms designs create an instrument that is narrower, more compact and typically more rugged than Porro prisms. There is also a difference in image brightness. Porro-prism binoculars will inherently produce a greater depth of field and a brighter image than roof-prism binoculars of the same magnification, objective size, and optical quality, because the roof-prism design employs silvered surfaces that reduce light transmission by 12% to 15%. Roof-prisms designs also require tighter tolerances as far as alignment of their optical elements (collimation). This adds to their expense since the design requires them to use fixed elements that need to be set at a high degree of collimation at the factory. Porro prisms binoculars occasionally need their prism sets to be re-aligned to bring them into collimation. The fixed alignment in roof-prism designs means the binoculars normally won't need re-collimation.
Prism Glass
Most optical prisms are made from boroscillicate (BK-7) glass or barium crown (BaK-4) glass. BaK-4 is the higher quality glass yielding brighter images and high edge-to-edge sharpness found in the more expensive binoculars. If the binocular specification does not clarify the prism glass type, then most likely it will be BK-7. Phase coatings are applied to roof prism glass to enhance resolution and contrast. Return to Top
OPTICAL COATINGS
Since some binoculars may have up to 16 air-to-glass surfaces, with light lost at every surface, optical coatings can significantly affect their image quality. When light strikes an interface between two materials of different refractive index (e.g., at an air-glass interface), some of the light is transmitted, some reflected. In any sort of image-forming optical instrument (telescope, camera, microscope, etc.), ideally no light should be reflected; instead of forming an image, light which reaches the viewer after being reflected is distributed in the field of view, and reduces the contrast between the true image and the background.
U.S. Navy binoculars
Reflection can be reduced, but not eliminated, by applying optical coatings to interfaces. Each time light enters or leaves a piece of glass; about 5% is reflected back. This "lost" light bounces around inside the binoculars, contirbuting to an image that may become hazy and hard to see. Lens coatings effectively lower reflection losses, which finally results in a brighter and sharper image. For example, 8x40 binoculars with good optical coatings will yield a brighter image than uncoated 8x50 binoculars. Contrast is also improved by good coating due to the partial elimination of internal reflections. Please note the different coating specifications that are available with todays optics.
Coated Optics - A single layer on at least one lens surface.
Fully Coated Optics - A single layer on all air-to-glass surfaces.
Multi-Coated Optics - Multiple layers on at least one lens surface.
Fully Multi-Coated Optics- Multiple layers on all air-to-glass surfaces.
Return to Top
MECHANICAL DESIGN
Focusing and adjustment
Binoculars to be used to view objects that are not at a fixed distance must have a focusing arrangement. Traditionally, two different arrangements have been used to provide focus. Binoculars with "independent focus" require the two telescopes to be focused independently by adjusting each eyepiece, thereby changing the distance between ocular and objective lenses. Binoculars designed for heavy field use, such as military applications, traditionally have used independent focusing. Because general users find it more convenient to focus both tubes with one adjustment action, a second type of binocular incorporates "central focusing", which involves rotation of a central focusing wheel. In addition, one of the two eyepieces can be further adjusted to compensate for differences between the viewer's eyes (usually by rotating the eyepiece in its mount). This is known as a diopter adjustment. Once this adjustment has been made for a given viewer, the binoculars can be refocused on an object at a different distance by using the focusing wheel to move both tubes together without eyepiece readjustment.
There are also "focus-free" or "fixed-focus" binoculars. They have a very deep depth of field from a relatively large closest distance to infinity, and perform exactly the same as a focusing model of the same optical quality focused on the middle distance. Most modern binoculars have hinged-body construction that enables the distance between eyepieces (Interpupillary Distance) to be adjusted to accommodate viewers with different eye separation. This adjustment feature is lacking on many older binoculars.
Image Stabilization
Shake can be greatly reduced, and higher magnifications used, with binoculars using image-stabilization technology. Parts of the instrument which change the position of the image may be held steady by powered gyroscopes or by powered mechanisms driven by gyroscopic or inertial detectors, or may be mounted in such a way as to oppose and dampen sudden movement. Stabilization may be enabled or disabled by the user as required. These techniques allow binoculars up to 20× to be hand-held, and greatly improve the image stability of lower-power instruments as well. There are some disadvantages: the image may not be quite as good as the best unstabilized binoculars when tripod-mounted, stabilized binoculars also tend to be more expensive and heavier than similarly specified non-stabilized binoculars.
Alignment
Well-collimated binoculars, when viewed through human eyes and processed by a human brain, should produce a single circular, apparently three-dimensional image, with no visible indication that one is actually viewing two distinct images from slightly different viewpoints. Departure from the ideal will cause, at best, vague discomfort and visual fatigue, but the perceived field of view will be close to circular anyway. The cinematic (what you saw at the movies) convention used to represent a view through binoculars as two circles partially overlapping in a figure-of-eight shape, which is not true to life.
Misalignment is remedied by small movements to the prisms, often by turning screws accessible only inside the binoculars, or on certain binoculars, by adjusting the position of the objective via eccentric rings built into the objective cell. RCS Optics recommends that you seek out a professional to have any realignment adjustments made. In most cases, trying this yourself will void the warranty and in addition, the nitrogen gas purge will be lost and the likelyhood of moisture and fogging problems will increase. If you happen to have an alignment problem with an existing pair of binoculars email us at customerservice@birding.rcsoptics.com providing us with your binocular brand and model and we will be glad to assist you. Return to Top
FIRST-TIMER BINOCULAR RECOMMENDATION
Magnification: Select a magnification between 6 and 8 power. Remember that eye relief and field of view will get smaller as the magnification is increased.
For birding we suggest a magnification of 8x. We also recommend larger objective lens (40-56mm). This will mean a heavier pair of binoculars though.
Field of View: Select a pair with a wide field of view.
Eye Relief: Check for an eye relief measurement of at least 15mm if you intend to wear eyeglasses while using the binoculars.
WaterProof: Select a pair of binoculars that is waterproof and fogproof. High quality models will also have the inside cavity of the binoculars
purged with nitrogen.
Birders should keep the following features in mind. Color and Clarity are two very important features that will make the difference between just an okay and a great birding expedition. You cannot begin to imagine the difference until you have seen for yourself. Larger objective lens will gather more light when it really counts- at dusk and dawn. Close focus should be realativity low, say 8 to 10 feet. RCS Optics is an authorized dealer for several high quality optics manufacturers. Contact us at customerservice@birding.rcsoptics.com if you have any questions. Return to Top
PROPER BINOCULAR USAGE
1. Adjust Eye Cup
The eye cup helps to postion the eye at the proper distance to match the eye relief of the binoculars. If the eye cup is rubber then roll the rubber lip so that the eye cups extend. Some binoculars are equiped with an adjusting ring that will extend the eye cups. Leave the eye cups retracted if you intend to wear eyeglasses while using the binoculars.
2. Adjust Interpupillary Distance
Adjust the barrels of the binoculars so that the view you are seeing becomes a perfect circle.
3. Focus and Diopter Adjustment
If you intend to wear eyeglasses or contacts or have 20/20 vision then set the diopter reference to zero. For the rest of us, first close your right eye and look at a distant object and focus using the central focus wheel until it is in sharp focus. Then close your left eye and open your right eye. Now adjust the diopter ring until the right eye is in focus. Note the diopter position for future reference. With the diopter properly adjusted, you may now view any object and it will come into sharp focus for both eyes simply by turning the central focus wheel.
NOTE: If you are using a zoom binoculars first set the magnification to maximum before adjusting the diopter setting.
4. Locating the Birds
While birding, first locate the bird with the naked eye, noting close landmarks. Then bring the binoculars up to face and use landmarks to help locate the bird. Return to Top
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