This is an essential guide to help you choose a pair of binoculars ideal for sailing, or any other water-based marine activity on a floating vessel.
The short answer to choosing a decent pair of binoculars for sailing is to make sure it is rated as 7x50 and waterproof, but we will examine all of the features you should consider. The size 7x50 means that the binoculars have 7x magnification and the objective lenses are 50 mm in diameter. In and around this recommended size is ideal for sailing or marine use because it has conservative magnification (necessary when on a wobbly vessel), large objective lens size (for low-light conditions), and generous exit pupil size -- all of which we will go over below.
On a moving vessel where you and the vessel are unstable, 5 to 7 times magnification is enough. It is tempting to buy 10x magnification because it sounds good to see farther but this too much on an unstable boat (shaky hands combined with shaky boat means anything past 7x is going to be a pain to use).
Objective Lens Size
You will often be boating in very low-light conditions (sunrise, sunset, and cloudy days) and so you will want to allow as much light into the lens as possible into the binoculars while still being practical to use. In and around 50 mm is the optimal size -- it allows a lot of light, has a large exit pupil (the size of the image at the eyepiece, especially important on a moving vessel), and a generous Field of View. Binoculars with objective lens sizes smaller than 50 mm will be more challenging to use in low-light conditions and binoculars with larger than 50 mm will add to the total weight.
Field Of View
The Field of View (FoV) of a pair of binoculars depends on its optical design and in general is inversely proportional to the magnifying power. It is usually notated in a linear value, such as how many feet (or meters) in width will be seen at 1000 yards (or 1000 meters), or in an angular value of how many degrees can be viewed (each degree of view is equal to 52.5 feet at 1,000 yards or 17.5 meters at 1,000 meters). Most 7x50 marine binoculars have fields of view in the 7 degrees neighborhood. However, some pairs are relatively narrow at less than 6 degrees and others are as wide as 8.3 degrees. With a larger FoV there is less scanning which makes it easier to find what you are looking for.
Often you will see the specification for Apparent Field of View (AAoV) or Apparent Angle of View (AFoV), the angle of the magnified field when you look through the optics -- this is an approximated figure by multiplying the actual FoV by the binocular’s power. Binoculars with AAoV greater than 65 degrees are considered "wide angle".
It is very desirable to ensure that your binoculars are fog-proof. In order for binoculars to be fog-proof they must be purged with nitrogen, to avoid moisture -- this also helps with corrosion-resistance.
The best rating system we currently have available is the International Protection (IP) Code System (International Protection Marking). The water-proof rating that you should be looking for is at least IP67 (or IPX7), as this rating means that the radio can be submerged in water up tp 1 meter (metre) in depth. IP68 (or IPX8) is preferred but not necessary for marine binoculars. To simplify the understanding of water-proof ratings and learn more about this topic, we have written extensively about it in a guide to understand waterproof ratings.
Since the operation of the binoculars will be over water you will want some kind of floatation, either the binoculars themselves float or you should attach a floating strap.
If you wear eyeglasses then you will need at least 15 mm of eye relief. Many binoculars offer diopter adjustments to allow eyeglass-free viewing or collapsing eyecups to accommodate eyeglasses if you want to leave your glasses on. Be careful with eye relief as it can reduce FoV.
Many marine binoculars, have a built-in compass that projects a magnetic bearing into the image that is projected onto the eye. Some of today’s marine binoculars feature digital compasses. Some compass systems are illuminated and some are not.
This is not a strong consideration because some marine binocular compasses will only work in the northern hemisphere and it increases the cost.
Several marine binoculars include a rangefinder reticle that consists of a vertical scale and horizon line projected across the image. When viewing an object of a known height (many fixed navigational aids and landmarks have heights printed on nautical charts) you can use the scale to determine your distance from the object using simple geometry. Some range-finding binoculars feature a laser system to determine distance accurately.
This is not a strong consideration as you may not want to do calculation to determine distances, and you should stay away from laser as they are better for hunting and not good at larger distance used in sailing.
- more powerful binoculars than 7x50 can be used if they come with image stabilization but that comes at the expense of money (they are more expensive), weight (require a battery), and maintenance (they are electronic and have more parts than can break down) -- if the battery dies the powerful binoculars become difficult to use with their powerful magnification
- if you have a very stable boat then 8x42 binoculars are good, such as these:
- if you want to save on size and weight then 7x35 and 7x42 binoculars are good