Central focus binoculars allow both eyepieces to be focused at the same time for faster focusing. Central focusing is made possible with a single focusing wheel that is found in the middle of the binoculars.
Binocular Lense Coatings
Lens surfaces are coated with super-thin coatings to limit reflections. Without a coating, you can lose up to 50% of the light entering the binoculars making images appear hazy with a low contrast. Binocular lenses using the best multi-layered coatings can offer up 95% of the light making images brighter and clearer.
Terms and symbols that describe binocular coatings:
(C) = coated optics: A single layer on at least one lens surface.
(FC) = fully coated: A single layer on all air-to-glass surfaces
(MC) = multi-coated: Multiple layers on at least one lens surface
(FMC) = fully multi-coated: Multiple layers on all air-to-glass surfaces
When we speak of collimation we are talking about the alignment of the optics as well as the mechanical alignment. You can actually feel when a pair of binoculars is "out of collimation" because after extended use, your eyes are fatigued and feel strained. You'll find that the more expensive binoculars are very well collimated whereas the cheaper models may be less so.
The term, "objective lenses" refer to the large lenses at the front of the binoculars. Binoculars with larger objective lenses (diameters) can gather more light which impacts on image brightness. Astronomy, for example, requires that you gather as much light as possible. In this case you will want to find binoculars with a large objective lens.
Binocular Magnification - The Meaning of 8 x 40 and Similar Measurements
All binoculars will display numbers that are indicative of the magnification power they offer. The first number refers to how many times bigger the image will appear. The second number relates to the diameter of the front lenses in millimeters. This number gives a value to the binoculars' ability to gather light. The bigger this second number gets, the larger the binoculars get (and heavier). The larger size also means better performance in dim light settings.
Roof Prism or Porro Prism Binoculars
There are two types of binocular designs that are differentiated by the kind of prism system used - a Roof prism or a Porro prism. Roof prism binoculars tend to be more lightweight and compact while Porro prism binoculars have a slightly better stereoscopic image.
But today's high-end binoculars of each design are considered fairly equal in optical quality. Preferring one over the other is a basically a subjective call.
The purpose of prisms in binoculars is to invert the image you are looking at and it is the quality of the prism that will determine the clarity of the image.
Prisms are referred to as being either BAK-4 or BK-7. A BAK-4 prism's quality is considered the finest because of the quality glass used. Better quality means sharper and well defined images. When the lenses and the eyepieces are in line with each other, they are the roof prism design. If they are offset from each other, they are the Porro prism design.
Eye relief is determined by how far back your eye can be from the eyepiece and still be able see the entire image. Eye relief can be anywhere from 14 to 20 mm. If you wear eyeglasses, you should test that the eye relief is sufficiently long enough to work with eyeglasses.
The Field-of-view relates to the horizontal width of the image you are looking at. In the linear form, it is expressed as the width in feet that one is able to see at 1000 yards. In its angular form it is typically between 5 and 8 degress. To convert the angle to the linear form which is expressed in feet, you need to multiply the angle by 52.5. Using a A wide field-of-view eyepiece will help reduce eye stress.
Image Stabilizing Binoculars
Image stabilizing (or "IS") binoculars are very effectivein lessening distortion when used in moving environments like cars or boats. High magnification binoculars with an image stabilizing feature can be used as hand held binoculars limiting distortion from hand movements. The main problem with image stabilzation binoculars is their increased weight.
Binoculars with internal focusing do not need to have the eyepieces adjusted for focusing. This adds to the durability of the binoculars as well as making them more water resistant.