ANGLE OF VIEW
Measures the width of a view in degrees, (see field of view).
As with camera lenses, coatings reduce light loss (and flare) caused by reflection of light from a glass-to-air surface. This can take place as easily within the barrel of the binocular as from the surface of the Objective. Good coatings increase light transmission and contrast. They cannot be identified by appearance or apparent color. As a general guide:
– one or more of one or more surfaces or lenses coated. Light loss up to 35%.
– all air-to-glass surfaces are coated. Light loss up to 17%.
– one or more of one or more surfaces or lenses coated with multiple layers of chemical film. Light loss up to 11%.
– all air-to-glass surfaces have multiple layers of chemical film. Light loss up to 5%.
(Note: These figures are not absolute and should be considered as guides only).
The mechanical alignment of the optical elements in a binocular. Both right and left hand optical axis must have proper orientation and location within the binocular barrel and must be parallel to each other in a quality optical system. High quality mechanical construction will ensure that the lenses and prism blocks maintain their correct alignment and provide years of comfortable viewing without headaches or eyestrain.
Measured in mm, this is the diameter of the beam of light leaving the binocular eyepiece, and determines how much light the eyes receive. Larger exit pupils provides brighter images. To calculate Exit Pupil, divide the objective size by the power. For example, the exit pupil of a 7×42 binocular is 42 divided by 7, or 6mm. The square of the Exit Pupil is the geometrical luminosity factor. Since the pupil of the human eye can shrink to a diameter of 2mm to 4mm on a sunny day, this specification is of greater import for low-illumination use.
Measured in mm, it is the actual distance, your eyes could be from the surface, of the eyepiece and still see the full image.
FIELD OF VIEW
Is the width of the area (in degrees) that can be seen at a given distance (usually measured at 1000 yds). Sometimes called, “Angular Field”, a large field of view permits you to see a large area at one time, and it also enables you to follow a moving subject with ease. The smaller the magnification the greater the field of view.
The distance between the center of the exit pupils. Usually adjustable.
Also called “Power”, this number reflects how many times the image is magnified. For example, through a 10x binocular the image will appear 10 times larger than with an unaided eye, an 8x will magnify only 8 times.With 8x power, an object which is 800 feet away will appear as if it was only 100 feet away.
Basically 1/2 a binocular with the center hinge removed. Primarily valued for their light weight and compact size.
Light amplification (up to 35,000x) permitting the viewer to see throught the binoculars or monocular in near total darkness.
The larger, or front lenses. The Objective Lens Diameter is the size of the outer (front) lens in millimeters. A 7×35 binocular has a 35mm objective lens. This helps determine how much light enters the binocular, although image brightness still depends on the size of the exit pupil. Doubling the size of the Objectives quadruples the light gathering capacity of the binoculars; for example, a 7×50 binocular has twice the light gathering capability of a 7×35 binocular and four times the light gathering ability of a 7×25 binocular (all else remaining equal).
Small, light-weight, low-power binoculars, often decorated with faux mother-of-pearl. These binoculars are usually available in 2.5x to 5x, and are often simple lenses cemented into the barrels with no prisms at all.
PRISM TYPE (ROOF or PORRO)
Erecting prisms are used in all binoculars to correct for inverted (upside down) images. Two types of prisms are common — porro or roof. In general, porro prisms yield greater contrast, but many roof prisms have a phase-shift coating, which can provide similar contrast. Porro prisms are available in two standard types: BK-7 and BaK-4. The glass density of BaK-4 provides superior performance. One modification of the Porro design is the Reverse Porro Prism, commonly used in compact binoculars to reduce overall size. Roof prisms are lighter in weight, more compact, and often more expensive. They can be readily identified because the barrel appears to be a straight tube. Porro prisms set the objectives further apart and may offer superior stereo imaging. Also, BaK-4 and roof prisms have a perfectly round exit pupil, whereas BK-7 prisms show a grey area on the outer fringe of the circle.
RELATIVE BRIGHTNESS (ALSO CALLED LUMINOSITY FACTOR)
Indicates the size of the light shaft that reaches the eyes, generally a brightness of up to 10 (approx) is good for daytime use, while from 10 -16 (approx) will be good at dusk or on a cloudy day with low contrast, and 25-50 (approx) for night time.
A high power monocular, often with interchangeable eyepieces which provide varied magnification, field or view, etc.
A numerical index, indicating a binoculars light-gathering ability, a factor of at least “16″ is important for bird watching and hunting early in the morning, late in the evening or surveillance in dim light. A binocular with a high twilight factor will gather more light to make a dim area appear brighter.
Additional features to consider when buying binoculars include:
- How the binoculars focus (one center adjustment versus separate adjustments for each barrel). Some models are “focus-free.”
- Whether the binoculars allow you to fine-tune the focus of one eye (also called diopter setting or diopter control) so that both eyes see an optimally sharp image.
- Armour — protective coating (usually rubber) around the outside of the barrels.
- Waterproofing. True waterproof binoculars are Nitrogen purged, ie., filled with inert nitrogen gas internally to make them impervious to salt water, internal fogging, etc. Terms such as fog-proof, rain-proof, or splash-proof do not mean the binoculars are truly waterproof.
- Zoom — as with camera lenses, some binoculars offer variable magnification. Often these are larger, heavier and more costly than non-zoom models.
- Close-focus distance. The nearest an object can be to the viewer and still be rendered in focus.
- Image Stabilization — microprocessor controlled vari-angle prism which continually adjusts to maintain a steady image and correct for hand shake or vibration.
- As with any lens system, binoculars can be subject to the same types of distortion as camera lenses. These include pin-cushion, barrel, comma, chromatic aberration, etc. Manufacturers utilize the same types of corrections to overcome distortion and some binocular models may include features like aspheric elements, low-dispersion glass, field flattener elements, and so forth.
- With these optics it is especially important to remember that image brightness is not the only consideration. It is possible to have a bright image, but one lacking clarity, contrast, or color saturation. When making a selection, keep your particular needs and intended use in mind. Binoculars suitable for day boating may prove less suitable for star-gazing, while optics optimized for low light use, may not be the best choice for naturalists.