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The Binocular Lenses

Objective Lenses

• Objective lenses are the large lenses that gather the light.  More light gathering capability means seeing more detail.  Since birds are more active when the light level is low, as it is in early morning and late afternoon, more light gathering capability means more quality birding time.

Prism Glass

• Prisms refer to the mirrors within the binocular.  There are two types of prism glass primarily used today.  They are BaK-4 and BK-7.  The labels refer to the element used to modify the composition of the glass used in the mirror.  "B" is Boron and "Ba" is barium.  The Bak-4 prism is better and more expensive.  BaK-4 is a high density glass and provides a sharper image.  The coatings used on the prisms are also of primary importance (see Coatings below).  Binoculars with BaK-4 prisms will usually say so in the specifications.  If BaK-4 is not listed then assume the prisms are BK-7.

Coatings

• Coatings refer to optical coatings applied to the prisms and lenses of the binocular.  Both prisms and lenses are coated.  The main reasons for using them are to reduce internal reflections and improve light transmission.  Optical coatings are extremely important to deliverig a sharp and bright image.  There are different types of coatings and they are applied as a single coat or in layers.  Each layer of coating improves the image, but also adds cost.

          Coated (C) - Refers to a thin anti-reflective coating (usually Magnesium Fluorite).  It is deposited on one or
          more of the lens surfaces.

          Fully Coated (FC) - Refers to at least one thin anti-reflective coating.  The coating is on both sides of the 
          objective lenses, both sides of the occular lenses, and the long side of the prism.

          Multi-coated (MC) - With this type of coating one or more of the lens surfaces have multiple coatings.

          Fully Multi-Coated (FMC) - This type of coating is usually applied to the top-of-the-line optics and refers to all
          lens surfaces having multiple coatings.  While this indicates greater care has gone into the design of the product,
          it does not necessarily guarantee the best quality.  Quality is determined by the way the total product has been
          manufactured.

Selecting a binocular is a highly individual choice, of course.  But go for the highest level of lens coating you can afford.  This is especially true if your eyes are susceptible to glare or you're approaching the age of 50.

The Binocular Body

Porro Prism or Roof Prism Design

• Porro prism binoculars are the familiar offset barrel style.  The roof prism binoculars are the sleeker straight barrel style that have been available more recently (see photo on page 1).  Porro prisms are more bulky, a little heavier and are harder to weather-proof.  Their advantage is a wider field of view (FOV) and they are less expensive so offer the same optical quality for less money.  The roof prism models are more tolerant of rough treatment with less chance of the lenses becoming mis-aligned.  Even though roof prisms cost more, their ruggedness may pay off in the long run making them more cost effective.

Weight

• Weight of binoculars can be a factor.  But is is probably one of the least important aspects of choosing them.  Today's construction materials are lighter and stronger.  Unless you are very experienced, most people wouldn't be able to tell the difference between a binocular that weighs 28 oz. or 32 oz. even after a long day of birding.  If you do your research you'll discover a range of weight specifications for binoculars of the same class.  A more important aspect than the binocular's weight, is it's balance.  One that is well-balanced puts very little stress of arms, wrists and hands.  A poorly balanced pair will cause more stress even if it is lighter in weight.

Ease of Focusing

• When it comes of focusing the key is speed and precision.  You don't want to miss the bird because you can't get it in focus quickly.  Ideally, you want the focusing mechanism to go from close focus to infinity in a full turn (360 degrees) with a variable speed focusing gear.  This mechanism allows focusing slower when the image is near, and faster when farther away.
• Equally important is the depth of focus.  Binoculars that have a shallow depth of focus will take extra effort to get a sharp image.

Eye Cups

• Eye cups help eliminate light that would affect our vision while using binoculars.  They also help to measure the distance from the occular lens to our eyes (discussed in Eye Relief on page 1).  Eye cups were first made of rubber.  To get eye relief when wearing glasses the eye cups were rolled up or down.  The downside was the breakdown from repeatedly rolling the eye cups.  The first advancement in eye cups were those that slide in and out.  The problem with this is the difficulty of keeping them in place.  The next improvement came with eye cups that twist up and can be left at any position.  The newest, and the best are those that have click stops at intervals in the up-down path.  The eye relief distance for each stop is marked on the eye cup.

Protection for Binoculars

• Weather-proofing - There is a range of weather-proofing available.  However, some binoculars offer no water-proofing at all.  Other choices are shower-proof, water-proof, or nitrogen purged.  The water-proof type of sealing is accomplished with "O" rings.  Nitrogen-purged water-proofing is a must for active birders whether they are in a humid climate or not.  Binoculars that are nitrogen-purged will also keep out dust, sand, or anything else that can get inside and harm the binoculars.
• External Protection - The synthetic rubber that offers binoculars external protection is known as armoring.  It protects the body of the binocular from banging and bumping and some corrosive elements.

The Cost of Binoculars

The price range of birding binoculars ranges from quite inexpensive to scary.  Simple, inexpensive binoculars can be purchased for as low as $50 or so.  This price level is where some birders begin and they still work in some instances.  If you think your bird watching interest will remain casual or your eyes are very young these simple binoculars may be all you need.  The next level ranges from $100 to $250 or so.  Many people find the binoculars in this price range satisfactory for a lifetime of birding even if they become slightly obsessed.

Binoculars in the $500 to $1,500 or so price range will bring bird images up close and personal with a degree of clarity that rivals those half-million dollar cameras used for Super Bowl Sunday.  A word of caution ... don't sneak a peek through someone's top of the line optics if you intend to purchase moderately-priced binoculars.  You'll never want to go back!

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