Binoculars are a boat essential - vital for identifying seamarks, interpreting lights at night, keeping track of other vessels, and also bird and dolphin watching.
But with models on the market ranging in price from about £20 to £2,000, how do you decide which is right for your boat?
Magnification
The accepted specification for yachtsmen's binoculars is 7 x 50. What does this mean? The first figure refers to the magnification: a factor of 7 produces an image as if the viewer were 7 times closer to the image.
So why settle for 7, when some binoculars have much more powerful magnification - wouldn't 10, for example, enable the user to see even more? The answer is that the greater the magnification factor, the smaller the field of view - and the more susceptible to shaking.
A sailing boat is a fairly unstable platform to begin with, and 7 times magnification has come to be accepted as the best compromise: enough magnification, with a usefully wide field of vision, and manageable for relatively stable viewing, even in difficult conditions.
The second figure, 50 in this case, refers to the diameter of the objective lens. This determines how much light is gathered to form an image. Again, there is a trade-off between clarity and field of view. 50 has come to be accepted, through experience, as the most practical compromise at sea.
Prisms
The field of view is also affected by the optical design of the binoculars. Designs with this level of magnification use a system of prisms: the light (and image) reaches the eye after being bounced between prisms, rather than directly from lens to lens, as with a telescope.
This makes binoculars easier to use than the traditional mariner's telescope, because being much shorter, they are much easier to hold steady.
The price of binoculars is determined by many factors including lens quality and construction materials, and one of the key differences between models is the system of prisms used.
The Porro prism system, patented in 1854, uses a double prism Z-shaped configuration.
The alternative is the roof prism system, one form of which was patented by Carl Zeiss in 1905, a refinement that allows narrower, more compact (and more expensive) designs.
The roof prism allows objective lenses to be approximately in line with the eyepieces. Porro prism glasses have a wider, offset design.
Eye relief
Eye relief defines the distance from the eyepiece lens to where the image is formed: the distance the user's eye should be behind the eyepiece in order to see the clearest image. Eye relief is important for spectacle wearers to look out for, since their eye will be further from the eyepiece.
Binoculars with short eye relief can also be hard to use in conditions where it is hard to hold them steady.
Optical coatings
Binoculars can have as many as 16 air-to-glass surfaces, and light can be lost to reflection at every surface, reducing the clarity of the image. Lens coatings, which reduce reflection, result in a brighter, sharper image. The specification for binoculars may say “coated optics”, meaning one or more surfaces has an anti-reflection coating, or “fully coated,” meaning all the air-to glass surfaces are coated. Again, this will have a bearing on price.
Adjustment
Most binoculars have adjustable focus. “Independent focus” means that the two separate telescopes are adjusted independently. But for marine use, it is more common to have “central focusing” with a central focusing wheel, easily operated with one hand.
There may also be “dioptre” adjustment, allowing one of the two eyepieces to be adjusted to cater for differences between the user's eyes.
Less expensive “fixed focus” binoculars are designed to perform as well as an adjustable model focussed on the middle distance.
Most binoculars have a hinged construction that allows the distance between the eyepieces to be adjusted to suit the user.
Stabilisation
Image stabilising technology can be very helpful at sea, but stabilised binoculars tend to be both heavier and more expensive than non-stabilised models of equivalent optical quality.
Compass
One of the most useful refinements from a sailing point of view is the incorporation of a compass which can be read through the binoculars, enabling the user to take bearings on buoys, ships etc.
They don't do anything you can't do with a cheaper pair of binoculars and a separate hand-bearing compass, but they do it a lot more quickly and conveniently. Some binoculars also incorporate a range finder.
Waterproofing
Less expensive models are likely to be “water-resistant” whereas more expensive models will be properly waterproof. This means they should not suffer condensation and fogging in a hostile marine environment. Binoculars may also be nitrogen- rather than air-filled, to keep out moisture.
Construction
Binoculars may have an exterior of rubber, metal or plastic. Rubber is probably most resilient to the rough and tumble of life afloat, protecting both the binoculars and the boat itself from potential knocks. Look too for a chunky, non-slip finish which will make the glasses easy to use with wet or gloved hands.
Conclusion
How much do you want to pay? Having had a pair of really old fashioned, basic binoculars on board since we started sailing, I have been really impressed with the clarity and ease of use of friends' more luxurious models: stabilised and with integral compasses. I would be delighted if Santa Claus brought me a pair for Christmas. But I would not like to have a pair so precious I was afraid to use them!
There's no real safety issue here: the basic model will do the job very nearly as well as the most expensive (though it won't come with such a long warranty, or such a nice case and strap).
The following models are not intended as TheMainSail recommendations (we have not had the opportunity to test them all), but they have been chosen to illustrate the wide range of options available.
What's good enough for the RNLI is good enough for most of us, and the Bynolt Searanger II as used by the RNLI, is fully waterproof with a built-in damped Suunto compass - and it actually floats, too. It looks like good buy at £170.
If budget is a big consideration, Plastimo's Fixed Focus model at £30 claims to be easy to use, with impact-resistant armouring and non-slip grip.
If money's no object, Steiner Admiral Gold at £1,500 is said to be the brightest 7x50 binocular made, thanks to a new 3-D optical system claiming better reduction of stray light combined with a revolutionary optical coating. Waterproof, with a 30-year warranty, this military specification model also offers long eye relief.
Alternatively Steiner's Commander V (£550) also promises an exceptionally clear view, with an extra-large high performance compass, covering considerably less of the viewing area than earlier models.
At a more accessible price (£230) Bushnell's Marine model promises fully multi-coated optics. 100% waterproof/fogproof, it is O-ring sealed and nitrogen-purged to keep out moisture and highly resistant to saltwater corrosion. It also features an internal rangefinder and illuminating compass.
Mailspeed Marine's Visionary Classic binoculars are “full size functional” at just £20, complete with tough rubber armour body, coated lenses, centre focus with individual eye adjustment.
