The BIG FIVE of Marine Binoculars  

For all that has been written about what constitutes a "marine binocular," the factors most frequently considered are totally subjective. Since every square inch of the objective lens gathers the light of nine fully dilated eyes, any binocular -- large or small, expensive or disposable, waterproof or not - can be used as a marine binocular. Of course, some are infinitely more suited for life at sea than others and there are five features to consider before making a purchase. Since binoculars come and go, and specifications change even more frequently, the features to be considered are being presented without regard to make or model.

Choose a Binocular with Adequate Light Grasp

Light grasp is a term used to describe the amount of light that can enter an objective lens at any instant in time and is determined by the size of the lens. The larger the objective, the greater the light grasp and the brighter the image. To a point, this means the bigger. However, realities of weight, cost and inconvenience set in quickly in binoculars having objectives larger than 50 millimeters.

Choose a Binocular with Optimal Magnification

Due in part to the western world's generalization that bigger is always better, many of us have come to require magnification or "power" in a binocular over all other considerations. This is always a mistake - especially in the world of the professional mariner. The magnification of a binocular should never be higher than that required to do the job for which it was selected. The reason for this is that as magnification increases:

* image brightness decreases because the available light is spread over a greater area,
* the field of view decreases; making objects harder to find and keep centered, 
* motion becomes more pronounced,
* image degrading atmospheric disturbances are accentuated.

It is no wonder then that the most common marine binocular in production today is the 7x50. During World War II, the 7x50 earned the name "night glass" for its ability to offer as bright an image as could be obtained in a reasonably sized binocular.

Choose a Binocular with Good Image Quality at the Edge of the Field of View

Field of view - normally measured in degrees for marine binoculars - is a product of magnification and the size and position of field stops and has nothing to do with the size of the objective lens as many suppose. All other aspects being equal, the greater the magnification, the more narrow the field of view. Thus, those who go shopping for a "high powered" binocular with a "wide field of view," will always be disappointed. You should, however, keep in mind that what constitutes a "wide" field is subjective and has as much to do with marketing as it does with optics. It is not uncommon to see a 10x50 binocular advertised as having an "Extra Wide" field while a 7x50 by another manufacturer sports no such verbiage, even though its field is considerably wider. For any binocular, a wide field of view is preferable. However, in a binocular used at sea, having a good image at the edge of a 5-degree field can be more important than a poor image at the edge of a 7.5-degree field.

[For the purpose of comparison, the "pointer stars" in the Big Dipper are separated by about 5 degree.]