In this area I am going to explain the basics of binoculars, what the different features do and the ramifications that can be expected when comparing different makes and models. At the same time, I am not going to get overly technical on how the various features work, just what they mean to you when making a decision on which glass is right for you.
There are two styles of construction that are used in the manufacture of binoculars in today’s market. They are Roof Prism’s and Porro Prism’s.
Roof Prism Binoculars
In roof prism binoculars, the prisms overlap closely, keeping the objective lenses, the prisms and the eyepiece in a straight line. The result is a light weight, compact set of binoculars that are easier for the manufacturer to build and effect complete waterproofing. In a roof prism glass the light is reflected through the prisms up to 6 times which sometimes reduces the clarity of the image seen. They are typically more durable than an equivalent porro model.
Porro Prism Binoculars
In porro prism binoculars, the objective lens is offset from the eyepiece and result in a larger, heavier set of glasses. Porro prism binoculars reflect the light 4 times. Porro prisms are more difficult to make waterproof due to the construction style.
What the numbers mean
Binoculars are identified with numbers such as 10X50. These stand for the magnification X Objective lens diameter in millimetres.
Magnification (Power)
With the 10X50 example above the object being viewed would appear to be 10 times larger than it would to the naked eye.
Objective Lens Size
The second number in the example – 10X50, is the diameter of the objective front lens in millimetres, that is 50 mm. Typically the larger the objective lens, the more light is allowed to enter the binocular resulting in a brighter image. This is however not always the case. A high quality glass with a smaller objective lens that is coated with proper anti-reflection lens coatings can provide a brighter image than one with a larger objective lens that is not quality coated. The best way to determine what is best for you is to look through both and compare.
Exit Pupil
Refers to the size of the circle of light exiting the eyepiece of a binocular. The larger the exit pupil, the brighter the image. To determine the size of the exit pupil, divide the objective lens diameter by the power – a 10X50 binocular has an exit pupil of 5 mm. This is important if you are going to use your glasses in low light conditions – brighter is better.
Eye Relief
Eye relief is the distance a binocular can be held away from the eye and still present the full field-of-view. Eyecups are used to adjust eye relief for those who use eyeglasses. Extended or long eye relief reduces eyestrain.
Field-of-View (FOV)
Field of View is the width of the image that you see when it is measured at a distance of 1000 yards. Generally, the higher the magnification, the narrower the field of view is. In real world terms, if most of your viewing is at a long distance, a narrow field of view is acceptable as you are able to key in on what you want to view. If most viewing is to be at short ranges, it is better to use a binocular with a wider field of view to facilitate finding the subject to be viewed.
Glass Quality
There are many available glass choices used in the manufacture of the various binoculars. The standard grades of glass provide good image quality in good lighting conditions. High grade glass often labeled as ED: extra-low dispersion glass, FL: fluoride glass, and HD: high-definition glass provide enhanced color fidelity and better resolution in low light situations. When deciding on a set of glasses it is always preferable to pick the highest quality of glass that your budget allows.
Lens Coatings
The image seen in the binocular dims as it is reflected through the various prisms which results in reduced image quality as light is reflected or lost. Anti-reflective lens coatings reduce light loss and give a brighter, high contrast image.
Types of Coatings
Coated – A single layer on at least one lens surface, usually exterior lenses.
Fully Coated – A single layer on all air-to-glass surfaces.
Multi-Coated – Multiple layers on at least one lens surface.
Fully Multi-Coated – Multiple layers on all air-to-glass surfaces.
Phase Correction Coatings
Phase correction coatings have been designed to correct the image in roof prism binoculars as they suffer from a small amount of image distortion due to their construction style. These coatings offer the highest image contrast and resolution available and typically resolve the distortion issue.
Binocular Adjustment
It is a simple process to correctly adjust binoculars to fit your eyes and give you a clear image that is easy to view and produce no eye strain while viewing for long periods.
Most binoculars have a main adjustment wheel and a dioptre adjustment. The dioptre is usually on the right barrel and can be adjusted with either a ring on the barrel or a wheel attached to the main focus wheel. Start by covering up the right objective lens or close your right eye, look through the glasses and focus your left eye to the clearest image you can. The cover the left objective lens or close your left eye and adjust the right eye to a clear image with the dioptre adjustment (without moving the main focus adjustment). Test with both eyes to ensure clarity with no eye strain.
Those binoculars that claim to feature automatic adjustment should be avoided as they really depend on your eye responding to changes and result in increased eye strain over long glassing sessions.
Waterproof/Fog Proof
If you are going to use your glasses in rain or snow you should pay careful attention to their waterproof properties as they vary with the wording.
If the glasses do not state that they are waterproof, assume they are not.
Water Resistant or waterproof glasses typically are equipped with O-ring seals, but are not nitrogen or argon gas purged which leaves room for internal condensation in inclement and/or cold weather.
Fully Waterproof Glasses are O-ring sealed and nitrogen or argon gas purged for total waterproof and fog proof protection. These models can withstand complete immersion in water and stay dry inside. The interior optical surfaces won’t fog due to rapid temperature change or humidity as both argon and nitrogen are moisture free.
Collimation
Collimation refers to the alignment of the two barrels of the binoculars. If the left and right barrels are not aligned correctly you will experience eye strain, especially if you are spending a lot of time glassing. The eyes try to compensate for the difference in positioning of the two images and it is almost like going cross-eyed for an extended period of time which results in sore eyes and possible headaches. If you suspect your glasses are out of collimation, put your glasses up to your eyes and slowly pull the glasses away from your face. The two separate circles that you see should be aligned, that is they should appear in the same planes as they separate and not have one higher or lower than the other. If you suspect your binoculars are out of collimation, return them to the manufacturer for proper alignment.
Rubber Armour
Rubber armour provides multiple benefits to you. It helps protect the binoculars from the bumps and scratches that come with day-to-day use. It provides a comfortable gripping surface, making them easier to hold. It allows easier cleaning of soiled binoculars and reduces the possibility of unwanted noise when they contact a metallic object.
