Light transmission is another critical factor in marine binoculars. In any given situation, the amount of light striking the objective lens of binoculars’ is the same regardless of the quality. The important factor here is how much of that light is passed through to the ocular lens. Less expensive binoculars may only allow 80% or less light transmission while Quality binoculars will allow 90% light transmission. The top rated marine binoculars will allow 95% to 97% of light to reach the ocular lens. This means brighter images when viewed and are superior for low light situations such as night time viewing.
On a bright sunny day on the water you can’t help but notice the reflected glare from the water’s surface. Lens coatings are designed to reduce this glare and improve the transmission of light resulting again in sharper and brighter images.
Coatings come in a number of different variations, from "Coated" to "Fully Multi-coated" with the latter being the preferred choice. Lens coatings will appear as subtle rainbow hues of colors on the lenses.
The use of prisms in binoculars allow for brighter images and higher magnification than non-prism type binoculars. There are typically (2) types of prisms used in the manufacture of binoculars today, Roof Prisms and Porro Prisms.
Roof prism binoculars result in a narrow more compact design. They can be easily identified by the fact that the ocular eyepieces are in line with the objective lens resulting in straight narrow tube design. Roof prisms employ silvered surfaces that reduce light transmission by up to 15% resulting in loss of image brightness.
Porro prism binoculars will inherently produce a brighter image than roof-prism binoculars of the same magnification, objective size, and optical quality while at the same time typically provide a wider field of view.
Another issue with prisms is the quality of glass used in the binoculars manufacture. The two most common are BaK-4 and BK-7. BaK-4 is the preferred type since it uses quality barium crown glass which produces sharper images by preventing internal light scatter.
Field of View (FOV) expresses the width of view at a specific distance, usually 1000 yards or 1000 meters. A field of view which is to narrow will result in difficulty in locating what you are looking for. A field of view which is too wide will result in image distortion at the edges.
The field of view on marine binoculars is typically engraved on the binoculars as either a linear value such as: 350 feet (or meters) at 1000 yards (or meters). It can also be expressed angularly such as 5° or 7°.
In an effort to compare apples to apples, to convert an angular FOV to a Linear FOV:
Look for marine binoculars that have a FOV of not less than 5° or more than 7.5° this should provide the average user a balance between a reasonable FOV and image sharpness.
If you are an eyeglass wearer and the binoculars’ individual eyepiece focus is inadequate to correct for your vision, Eye Relief becomes an important issue. Basically eye relief is the distance that you can hold the binoculars away from your eyes when you are wearing either sunglasses or prescription glasses and still retain image focus. If eye relief is an issue, you should choose a pair that offers a minimum of 16mm eye relief with 18mm + being better.
Many binoculars built for marine use come with an integrated compass that will enable you to take the bearing of an object while observing it. While I may not find much use for binoculars with a range finder scale, for me an integrated compass is well worth the extra cost. My old hand bearing compass has now been relegated to the bottom of the chart table drawer or occasionally used as a paper weight.
Looking for a distant mark and taking a bearing of that mark at the same time is a valuable tool for the navigator. This value becomes even more evident in low light conditions.
Some marine binoculars have a range finding scale built into the binocular. This feature has become quite common place especially in those binoculars that have an integrated compass. This allows the distance to an object to be estimated if the height of the object is known with some simple math thrown in. While this feature has some practical usage for a limited number of boaters, for me, after 40+ years on the water, I do not ever remember using this feature. My personal preference is if it comes with the binoculars I choose great, if not I am not going to miss it. Usually when I need to know the distance to an object, I fire up the radar!
Typically this is a heavy rubber compound that is molded to the outer body of the binoculars. Since things on boats tend to get bounced and tossed around much more than they do on dry land, this armor coating will prove invaluable. They not only protect the binoculars from getting banged up, they also help to prevent sliding when set on a wet surface.
This is primarily an individual preference. I personally do not put a lot of weight (no pun intended) to this; either for or against. Heavier binoculars may give you a more stable viewing advantage, lighter binoculars will allow long usage without feeling like you have brick tied around your neck. In most cases the average boater rarely wears their binoculars for any lengthy period of time, they are usually kept in a cradle or stowage box below or at the helm until needed so weight is usually a non-issue.
If your plans are to make use of your binoculars in other areas besides boating, then you may want to consider their weight when making your decision.