The size of the objective lens measured in millimeters. Aperture is shown on binoculars as the number after the X. For example, a pair of binoculars with an aperture of 42 would be designated by 10x42. The larger the aperture, the greater the amount of light the lens will allow in and the brighter the viewing subject will appear.
The process of light being split as it passes through glass or a prism. The lower the dispersion or angles at which the light is split, the sharper the colors will appear. Most binoculars have either low dispersion or extra-low dispersion characteristics.
If you hold your binoculars about a foot away from your face and point them at any light source, you will see a little dot in each eyepiece. That dot is the exit pupil and is typically measured in millimeters. The size of the exit pupil needs to be smaller than the dilated size of your pupil. If the exit pupil is larger than your pupil, then you won't get the maximum benefit of the light that is being captured by the binocular. Children's eyes can dilate more than adults eyes. More specifically, a child's eye can dilate to approximate 7mm as were an adults eye can only dilate to about 5mm.
The distance you can hold your binoculars away from your eyes and still get the full effect of the magnification and field of view. Longer eye relief can be helpful if you have astigmatism that must be correct with glasses. If you are nearsighted, you may be able to remove your glasses and refocus the binoculars to take full advantage of their capabilities.
The part of the binocular that your eyes look through. Eyecups can be hard material or more commonly made of soft rubber. If you use binoculars for star gazing it is nice to have eyecups that extend and wrap around the outer part of your eye socket to block any ambient light.
The area that you can see when looking through your binoculars. The field of view is impacted by both the magnification and the objective lens size (aperture). The field of view decreases as the magnification increases. Conversely, the field of view increases as the objective lens size (aperture) increases. For example, a 10x42 binocular has a smaller field of view than a 7x42 binocular. Also, the field of view of a 10x42 binocular has a larger field of view than a 10x36 binocular. Field of view is often expressed by the width of view at 1,000 feet.
Generally speaking, prisms are blocks of transparent material typically glass with the opposite ends having the same sizes and shapes. More specifically for binoculars, prisms are used to capture and manipulate light as it travels from the objective lens to the eyepiece. There are two types of prisms used in binoculars. These prisms can be referred to Porro prisms and roof prisms.
The Porro prism was invented by Ignazio Porro sometime in the middle of the 19th century. Binoculars that use Porro prisms will not have a straight tube, but rather will have a step in the middle of the tube where the two prisms are arranged side by side.
Roof prism binoculars have the prisms arranged on the roof of the tube. This design allows for a straight tube that provides for a narrower sleek design seen in most modern-day models.
In simple terms, chromatic aberration the inability of a lens to focus all colors of the light spectrum to a specific point. Chromatic Aberration manifests itself with fuzzy lines around the edges of objects, particularly where there is a transition from light to dark.
The objective lens is the large lens in the front of binoculars. The larger the objective lens, the wider the field of vision and the greater amount of light that is let in. Larger objective lenses are helpful in low light situations. They are also beneficial for viewing moving objects as they provide a wider field of vision.