Let's Talk Binoculars

By Dick Malnory


Are you in the market for new binoculars for birding?  Whether you are purchasing your first pair or "upgrading", here is a rundown on binocular features to help you choose the pair that works best for you.

What you want from a pair of binoculars for birding is

• Wide field of view
• A bright image
• Quick focus
• Eye relief

Let's look at optics specifications that you will be taking into consideration.

Prism Type:

Binoculars today come in two basic designs, Porro Prism and Roof Prism, which refer to the type and arrangement of optical prisms in the binoculars.

To compare, Porro Prism was the standard design until 1960.  It offers a wider field of view and is more light efficient.  It also offers more contrast, but Porro Prism binoculars are heavier and chunkier.

Roof Prism binoculars are more streamlined, lighter weight, more compact and easier to hold, but are more expensive.

Magnification:

The recommended magnification for most birding binoculars is 7x or 8x.  Higher powered binos give greater detail but are less steady, offer a narrowed field of vision, and also less depth of field.  Note - image stabilization binoculars are definitely recommended for higher power binoculars and for use by individuals who don't have steady hands!

Objective lens:

This is the lens closest to subject you are viewing.  When you see a rating for a pair of binoculars such as 8x40, 8 represents the magnification power and 40 is the diameter of the lens, in millimeters.  The larger the objective lens, the more light is available, giving a brighter image.  A general rule of thumb is that the objective lens should be 5 times the magnification power; with a magnification power of 8, an objective lens of 40 or more millimeters would be recommended.

Eye Relief:

This refers to the optimal distance between the binocular eyepiece and your eye.  Binoculars will have adjustable eyecups that either fold or rotate to accommodate eyeglass wearers.

Prism Glass:

The better the optical quality used in your binoculars, the better the light transmission.  There are 3 grades of prism glass - BAK4 - Best; SK15 - Better; and BK7 - Good.  If the specs for a pair you are considering says the prisms are made of eco-glass, that means no lead or arsenic was used in the manufacturing process.

Coatings:

Coatings are used to improve light transmission.  You will find one of these symbols on binoculars that have any coating - C - one or more optic surfaces are coated; FC - all air-to-glass surfaces are coated; MC - one of more surfaces are multi-layer coated; and best of all - FMC all glass surfaces are fully coated.

Field of View:

Refers to the width that can be viewed at a distance of 1000 yards.  The wider the field of view, the easier to find the birds.  The field of view will be expressed in degrees or actual feet that can be viewed at 1000 yards; 8 degrees is a very good field of view - equivalent to 420 feet.  With all other parameters equal, choose binoculars with the wider field of view.

Finally, consider the weight when choosing binoculars; a shoulder strap can distribute the weight better than a neck strap. Are the binoculars comfortable in your hands, and can you reach the focusing knob easily?  It's best, for this reason, to shop in person.  If ordering online you will want to make sure of the vendor's return policy.

NOTE:  Visitors at Hagerman National Wildlife Refuge who take tram tours and guided walks may use loaner binoculars provided by the Friends of Hagerman and help in adjusting them if needed - a good chance to try out a pair for those new to birding.