This is the width of a scene that can be viewed at a distance of 1,000 yards, expressed either in feet or degrees of arc. The greater the magnification, the smaller the field of view. If you increase the magnification from 7x to 8x, a buoy will appear larger and closer, but the smaller field of view will make it harder to scan the horizon to spot the buoy quickly. A wide field of view is better for following fast moving action or scanning for moving objects. Two types of prism systems are available.
Roof prisms place the lenses in line for compactness.
With porro prisms, the eyepieces are offset from the front lenses and generally provide better depth perception and contrast. Relative Brightness Relative brightness indicates how well the binoculars will perform in dim light, and is the square of the binocular's exit pupil diameter. Exit pupil diameter is the objective lens divided by the magnification. For 7 x 50s, that's 7.14. That's why 50mm objective lenses are so popular with boaters.