Magnification (Power) is determined by the size, curvature, and placement of the lens systems. The "7" in 7 x 50 indicates a magnification power of seven. This means that an image 700 yards away will appear to be only 100 yards away and seven times larger than with the naked eye. The "50" is the diameter of the objective (front) lens in millimeters. Most boaters prefer 7 x 50 binoculars because the 7x magnification allows you to locate objects and keep them within your field of view, and the 50mm lens admits enough light to make objects visible under low light conditions.
Using binoculars with greater magnification makes finding and holding objects difficult, especially on a pitching deck, because small movements of the binoculars translate into sweeping arcs in the field of view. Greater magnification also reduces the clarity and definition of the image. Larger objective lenses gather more light, but the trade-off is heavier, cumbersome binoculars.
Field of View, 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. Each degree equals 52.5 feet of width at 1,000 yards. 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.
