Prisms are used to condense the viewed image for maximum magnification in a short space. Prisms recreate an image of the original that is much purer than any image you would get from mirrors or multiple lenses which could magnify the image in a shorter distance. The resulting distortion would render the image impractical.

There are two kinds of prisms used in binoculars: roof prisms and Porro prisms. You can tell which type of prism is used from the shape of the binoculars.

Just-Binoculars Prism diagram

Porro Prism
Binoculars with Porro prisms have a jog in the light path that results in the eyepiece being much smaller than the barrel of the binoculars, as seen in this diagram:

Roof Prism

Roof prisms result in an exterior design of the binoculars that is more like two straight tubes. The roof prism design was perfected in the 1960s; binoculars with this design are compact and easier to hold. For a while after their introduction, roof prism binoculars required much greater optical precision, so they cost more to manufacture. The Porro prism rendered better contrast and was still favored by purists for a long time after the advent of the roof prism. The technology of roof prisms has improved greatly since their debut, which makes newer models a better value.

BAK-4 prism

The BAK-4 prism is made of a high quality glass and produces sharp images and good edge to edge sharpness. . Generally, higher quality binoculars will use BAK-4 prisms in the construction process. Phase coated prisms take it one step further, the coating process enhances the resolution and contrast of images coming through the binocular and are generally applied only on more expensive binoculars.

Here is a chart of how binoculars and the numbers work together in various applications    courtesy of just-binoculars. COM

 

Some Quality Binoculars

Steiner 7x50 Commander XP C Binoculars w/ Compass 395

Steiner marine binoculars were the first to bring a compass to the marine Boat Binocular. The XP optics Steiner claim, keep the images sharp even in poor light. The lenses are treated with a protective coating to resist scratching and repels water. Each pair has 3 eyecups so you can choose the right one for you. Waterproofing is to 10m and Steiner's auto focus system are among the many features. Retail price for the XP $1,300 compared to Steiner Commander V's $1,600

You could go with the Steiner Observer model which retails around $450 and is also very good and is reviewed in the Motor Boating review below.

One thing you will notice about Boating Binoculars is the large variation in prices for what looks like the same features. With Marine Binoculars the optics and coatings can add dramatically to the cost, but also add dramatically to the image especially in low light and reflection of the water. Using the Commander XP from Steiner, you will find the image is very clear and not blurred. So if price is not a concern this is a good marine binocular.

Cheaper Options from Steiner include the; Navigator model retail and Observer model

Nikon  Sports 7*50 marine binoculars w compass 

These nitrogen filled and O-ring sealed binoculars are 100% Waterproof & Fog proof. With the new UCC lens coating giving you 95% Light Transmission, BaK4 prisms and multicoated lenses, you can see what you need to see! The built-in compass is just another one of its fabulous qualities! $590 retail.

The Fujinon "SX" series Boating marine binoculars, every lens and prism surface is coated with a special new EBC process. The result is an overall brightness factor of 95% across the full visible spectrum. The benefit is outstanding vision in low light and at night, an especially important consideration in marine applications. To improve water and fog resistance, the housing is dry nitrogen filled and sealed at the factory. With compass the list is $1,160

Celestron 7x50 WP marine binocular with compass and rangefinder

This feature-packed marine binocular includes a compass, reticle, rangefinder and individual focus. Celestron 7x50 WP-IF/RC is Waterproof & Fog proof Porro Prism Binocular (7.0-Degree Angle of View). The 7 x 50 boating binoculars include BAK-4 prisms and multi-coatings for sharp images. Oceana 7 x 50 IF/RC Reticle/Compass Binocular belongs to Oceana Series from Celestron that features waterproof models for marine use.

The rangefinder is a useful tool for boaters allowing you to figure our range and size. Most rangefinders require you to know the size of the object such as a lighthouse height.

Bushnell 7 x 50 marine binoculars

• Bushnell marine 7 x 50 binoculars Features
• BaK-4 prisms for bright, clear, crisp viewing
  
• Multi-coated optics for superior light transmission
• 100% waterproof: O-ring sealed and nitrogen purged for reliable, fog-free performance
• Non-slip rubber armor absorbs shock while providing a firm grip
• Fold-down or twist-up eyecups
• Large center focus knob for easy, precise adjustments
• Longer eye relief

Swarovski 7 x 50 boating binoculars

SLC 7*50 B Roof Prism Binoculars, no compass. Women tend to find the roof prism easier to hold.

Handy, ergonomic and exceptionally versatile - the SLC binoculars by Swarovski Optik are among the most-used binoculars ever. But the SLC series can now offer a multitude of tangible benefits. SLC new - this means an innovative design with improved ergonomics - for greater versatility and functionality

The focusing is via a center wheel. There is another knob to provide dioptric correction between eyes. The process of correcting for your eyes is to look through the binoculars and close the right eye and set the focus for the left eye with the center focus wheel. Switch eyes and correct for the right eye with the dioptric knob. Expensive at $1,450 retail

Try them on and make sue they fit your face comfortably the grip also. Try them out in daylight and at night if relevant.

Coatings, If you look at water with the sun shining on it, you will notice the glare. Lens coatings remove glare and allow more light to enter the optics. The more coating the more expensive the Binoculars. Cheaper Binoculars do not have the coatings as better models ands this accounts for the wild variation in prices.

Focusing. Center Focus with a wheel,  The center wheel focuses at the same time for your eye. Or do they compensate for unequal vision. This is by means of a dioptric correction through the adjustment of one eyepiece, usually the right-hand eyepiece. 

What about size and weight. Do you want a pair that can fit in a pocket then the Roof Prism is more likely to work.

Marine binoculars with Rangefinders are also useful. To use a range finder, you divide the height of an object that you know by the points read on the reticle scale when you look through the binocular. Example: A lighthouse is known to be 150 feet high, and appears to be 15 points on the vertical scale of the reticle. 150/15 = 10, so the distance from your point to the lighthouse is 10,000 feet.

Are they Waterproof, do they have Flotation straps, which are very handy if they are dropped overboard.

There are many places to look online for binoculars its confusing. However its also a great source of information. The downside is you cannot try them on unless you already have tried them.

 

Binocular Terminology

if you see the term Exit Pupil, Luminosity Factor, De-Alignment, Dioptric Correction, Oculars, Eye Relief, Resolving Power, Interior Focusing and many other descriptions and wondered what they mean this is the place to find the answer. Binoculars Etc

 

Sources of marine binocular reviews

Epinions Binocular 7 X 50 reviews

Motor Boat review of 7 x 50 Boat Binoculars with Compass

Practical Sailor: Binoculars over $300

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