Prism binoculars. Convex lenses are used for both objectives and eyepieces. A wider field of view and high magnification can be attained than is the case with Galilean binoculars. An erecting prism system is incorporated in the optical path to rectify the image. Also, employing prisms has the effect of making the length of such binoculars shorter.
Two of the important measurements you see on binocular specifications are Field of View and Close Focus (defined below). Whereas power and objective lens size determine how big and bright your image will be, field of view and close focus determine how much of the world you’ll be able to see through your binoculars.
Eye relief defined . It's all in the design of the eyepiece. For any binocular, there is an ideal distance that your eye should be from the eyepiece. This distance is called the eye relief. If your eye is farther away from the eyepiece than the eye relief distance, you lose the outer edge of the picture.
Binocular Field of View. This spec tells you the width of the area (usually in feet) that you can view at a glance, 1,000 yards from where you stand. A wide field of view is best to find and identify objects such as birds. Usually a higher magnification power results in a narrower field of view.
Binoculars can range from 6-12 magnification or even higher, but powers from 8-10 are generally preferred for birding. There are pros and cons to both higher and lower magnifications, and birders should choose magnification based on what best meets their birding and eyesight needs.
Understanding binocular specs, such as magnification, objective lens diameter and exit pupil will help you narrow down which pair works best for your needs. Binocular Size Binoculars come in a variety of sizes (defined by the objective lens size) for various outdoor pursuits.
Porro prism binoculars and roof prism binoculars using the Abbe–Koenig roof prism do not use dielectric coatings because these prisms reflect with very high reflectivity using total internal reflection in the prism rather than requiring a mirror coating.