What is a Marine Binocular?
Various articles have appeared in the yachting press recently suggesting that centre focusing binoculars, (that's the kind with a wheel in the middle), are suitable and appropriate for use at sea. Now any binocular is better than no binocular, and so if this is the sort you have that's fine. But if you plan to go and buy a pair mainly to use at sea it's bad advice to choose a landsmans binocular and I'd like to tell you why.
Marine Binoculars have "single eyepiece focusing". This means that you first look at some distant object, then shut each eye in turn and focus each lens on the distant object. From that point on you NEVER have to refocus, and EVERYTHING from 20 metres to infinity will always be in focus. A recent yachting magazine article said about one of these binoculars, "Having to focus each eyepiece individually rather smacked of 1940's design", and elsewhere used this statement, "having to refocus both sides every time you use them is a nuisance".
Actually single eyepiece focusing is very up-to-date and used by all modern armies, navies, and knowledgeable yachtsmen everywhere. Additionally you NEVER have to refocus after the first time unless someone else has used them, and this situation is no different to a centre focus binocular (that has a dioptre adjustment on only one lens and which needs to be refocussed for a new user).
In my opinion single eyepiece focusing binoculars are ESSENTIAL at sea, and it is poor advice to use roof prism binoculars, or indeed any binocular with a centre focus wheel.
At sea, with the boat pitching and rolling, and targets possibly only being in view momentarily on the crests of waves it is important to have the entire depth of field in focus all the time. This can only be achieved with an individual eyepiece focusing binocular where everything from 20 metres to infinity will be constantly in focus. Having to refocus to bring different areas of the seascape into sharp relief, as you must do with a centre focusing binocular, adds immeasurably to the chance of missing a buoy, a man overboard, or any other small or indistinct mark.
The essential difference between mariners and landsmen is that the landsmen knows what he or she wants to look at before raising the binoculars to the eyes. The mariner may know what he or she is looking for, but usually only has a vague notion of the quadrant in which it might be found, and the distance it will be away.
For the same reason that depth of field is important the width of the field is also. The wider the field of view the fewer times the horizon will need to be scanned to pick up a target. Although it is sometimes expressed in degrees, the mariner will more usually find a FOV expressed in "metres at a thousand metres".
Graticule. Professional marine binoculars usually also have a graticule, (a vertical and horizontal scale), printed on one of the lenses and usually graded in a scale called MILS. Using this scale providing you know your distance from a target you can estimate it's height or length, or the other way provided you know it's height, (say a tower whose height is given on a chart), you can quickly estimate your distance from it. If the binocular says it has a range finder reticle (graticule) this is what it means, it is not an electrical read out. Our latest SeaPro has a revolving scale to allow you to work out these heights and distances without recourse to a calculator or a sharp pencil.
Two shapes. Marine Binoculars typically come in two different shapes, long tens tube designs (Monk, Binocom, Sea Ranger and Steiner Navigator Pro), and the more compact but heavier short lens tube designs (Steiner and our own SeaPro). The most vulnerable part of a binocular is the lens tube that sticks out in front, and so the longer tube varients are more susceptable to damage but cheaper to make because they can use smaller prisms. The shorter versions require bigger prisms and so are a little heavier and more expensive to manufacture, but less susceptable to damage and easier to keep trained on an object.
Specialist marine optics from Steiner (sold here), Fujinon, Bynolyt Sea Ranger (sold here), Binacom, Monk, our own-brand Century SeaPro (sold here), and others exist for a good reason, and are not an out-of-date throw-back to a forgotten age that can be ignored in preference to a selection of bird watching and tourist binoculars.
As a final thought, single eyepiece focusing binoculars are also available in smaller objective sizes, 6x30 and 8x30, and these smaller objectives not only have a place for the older yachtsmen whose retinas will not expand sufficiently to gain full benefit from a 50mm objective, but also with day sailors of any age who maybe sailing in open boats or trailer sailers where space is at a premium. However these smaller sized binoculars are not available with integral compass, and these days such is the volume of production that it's usually cheaper to buy a binocular with a compass than one without. Indeed the extra tooling cost to build our SeaPro without a Compass was not an economic proposition.
The biggest market for binoculars in the UK is bird-watching and that is why we see centre focusing roof-prism binoculars so much on the High Street. They are more easily suited to compact design and have the benefit of being able to focus down to 1.5 metres or less, allowing close observation of the visitors to our bird tables and being able to view insect life such as dragonflies. Indeed we have a pair in the kitchen and in the car, but I wouldn't take them to sea!
USEFUL TIP. Once you have purchased a pair of single eyepiece focusing binoculars and set them up for your own use, look at the dioptre scale on the eyepiece and try to remember your setting. Steiners are particularly easy to do this with because the scale is very easy to read. If anyone else has used them in the meantime you can immediately re-adjust to your personal setting without having to go through the focusing bother again and again. It quickly becomes a habit and also works sailing with someone else, provided you know your settings you can use their binoculars instantly too.
John Church