Choosing and Using Binoculars

By Julian Parks

Safety #

SOLAR OBSERVING: DANGER! NEVER LOOK AT THE SUN WITH A BINOCULAR. IT WILL PERMANENTLY BLIND YOU. Use the proper solar filters that fit over the objectives of the binocular. Also do not project an image of the sun with binoculars. The heat of the sun can crack the eyepieces in binoculars. Some eyepiece designs are cemented together.

Wear the Binocular Neck Strap at all times when using your binoculars. Tell your friends and family to do so as well. This will keep you and someone from dropping your binocular and breaking it.

Introduction: Demonstration of Binoculars #

Parts of the Binocular #

  1. Objective
  2. Eyecup
  3. Diopter Adjustment Scale
  4. Body
  5. Prism Cover
  6. Center Hinge
  7. Bridge
  8. Center Focus Wheel
  9. Objective Support
  10. Interpupillary Distance Scale

Choosing Binoculars #

Age #

Age is the single most important factor in choosing a pair of binoculars. Although with age a person’s exit pupil tends to get smaller; that is, 5mm after age 50, one should determine his or her exit pupil. This is usually done by trial and error. If you do it in the dark, you cannot see what you are doing when holding up a millimeter ruler to your eye in front of a mirror. You can try a dimly lit room however. Sky Publishing use to sell a device for measuring one’s exit pupil.

Another factor affecting your exit pupil is your observing environment. If you observe from the city or the suburbs, light pollution will affect your exit pupil and will not allow it to fully dilate as if you were under a country sky. For instance, if you observe with a 7 x 50 pair of binoculars under a city sky, this is like observing with a pair of 7 x 35 binoculars. Why? Because your pupil is not fully dilated to take advantage of the extra exit pupil or circle of light being projected by the 7 x 50 binoculars. What is the exit pupil of a 7 x 50 binocular? Answer. 7.1mm. Divide 50mm the aperture of the binocular. In this case by the power of the binocular 7x. The 7 x 35 binocular exit pupil is 5mm.

Also when buying a pair of binoculars checkout the eye relief or how close you have to hold your eye to the binocular to get the full field of view. Some eyeglass wearers require a long eye relief. Usually eye relief of 15mm or more is sufficient for eye glass wearers and anyone else.

Mechanical Design #

It is 10 times harder to make a good roof prism binocular than a standard porro prism one. A roof prism binocular can equal, but never exceed an excellent quality porro prism binocular. A roof prism binocular is also much more expensive than a porro prism binocular due to the special prism and phase shift coatings used for this design. Of course, it does not mean a well made roof prism binocular is not good for astronomy. It is just more expensive due to the high standards required to make one.

Most roof prism binoculars are made in Germany. Porro prism binoculars are made in Japan. Whatever design you choose make sure the binocular has Bak-4 prisms and the lens is fully multi-coated. Also because it is a well known European brand does not mean it is safe to buy that binocular. Some binoculars are shipped to Germany from Korea with no country of origin marked on the binoculars. When said binoculars left Germany they were then stamped as being made in Germany. On a similar note, if buying binoculars made in either the former Soviet Union or China you might be getting a good deal, but buyer beware. If you can, try them out first and make sure the dealer has a liberal return policy.

Also concerning zoom binoculars stay away from them. Zoom binoculars show up in greater numbers for repairs than any other design. They were originally developed as a “gimmick” to encourage sales simulating the success of sales of zoom camera lenses.U.S. military specification design is still the best design for binoculars, although not as popular as the center focus design. If someone is advertising their binoculars as “military design binoculars, just make sure they are U.S. military design and not someone else’s military. U.S. military specification design means that each eyepiece has to be focused for each eye, that the barrels of the binocular are sealed or water proofed, and filled with nitrogen, prisms are Bak-4, and the lens are fully multi-coated.Center focus binoculars are not usually water proofed and filled with nitrogen. You achieve focus with a center focus wheel and the right eye piece for the right eye. For a good center focus binocular make sure the prisms are Bak-4 and the lens are fully multicoated.

Also make sure the center wheel for focusing does not feel mushy and does not loose focus. If you have to continue to refocus then something is wrong with the center focus wheel. Also look for any excess grease around the center focus wheel and the eyepiece diopter used for focusing. If you see grease leaking out from either do not buy.

Center focus binoculars being more fragile and not water proofed are more susceptible to damage than a U.S. military specification design.

Concerning the image stabilized binoculars from Canon, these are excellent binoculars for astronomy and do not require a tripod. They are just expensive. The 15 x 50 is a good choice, because of the wider field and ease of use than the 18 x 50 model. Also even though they do not require a tripod, better to put them on one. The later models do come with a standard threaded hole for a tripod. By using a tripod, you free your hands up to take notes or read a star chart without having to go back and find what you were looking at.

Optics #

Any good designed binocular, whether the U.S. specification design or the center focus design, will have Bak-4 prisms, fully multi-coated lens, air-spaced objectives, and nitrogen filled tubes sealed with o-rings to water proof the binocular.Bak-4 prisms, fully multi-coated optics, and air-spaced objectives allow for better light transmission, and therefore, a better view. Air-spaced objectives allow for better resolution. The nitrogen filled tubes sealed with o-rings keeps water, whether of the liquid variety or the vapor variety, from entering the binoculars system and causing mold or mildew to grow.

Collimation means that when you look through a binocular the view is perfectly comfortable with no strain or “uneasy” feeling.” More to the point, the lenses’ optical axes are not lined up with the mechanical axes of the tubes correctly.

For a quick in store collimation test do the following:

  1. Spot an object 50 to 100 yards away and focus for both eyes.
  2. Now look at the target and take the binocular down and let your eyes relax for half a minute.
  3. Now look at the target again, but cover the left objective with you hand.
  4. Now pull your hand away from the left objective.
  5. If the target is out of focus, but quickly goes back into focus, your eyes are adjusting to the inherent error in collimation, and the binocular should be rejected.

Field of View is expressed as feet at a thousand yards. This is fine if you are in the artillery, but astronomers use degrees to define the field of view. If you see on your binoculars a field of view 316 feet at a thousand yards, it means the field of view is 316 feet from edge to edge in your binoculars. To convert this to astronomical field of view or degrees divide by 53. In this case 316 divided by 53 equals 5.96 or 6.0 degrees field of view.

Purpose #

Next when purchasing a binocular you must consider the purpose or what are you buying the binocular to observe-star gazing, sports, and birding. What? Binoculars are great for doing a variety of things such as an accessory to the telescope to star hop to those deep sky objects you want to find or an asteroid. Also binoculars are great for learning the constellations, following sunspots across the sun(DANGER-BE SURE TO USE THE PROPER SOLAR FILTERS), the motion of the planets among the constellations, the phases of the moon, sky conditions, comets, variable stars, and nova.For astronomy the 7 x 50 and the 10 x 50 have been the traditional choice. For your first pair of binoculars, get this standard size over the giant binoculars. The 7 x 50 or 10 x 50 is good for general purpose viewing and portable. The 7 x 50 is good for dark country skies. The 10 x 50 is good for the city or the suburbs due to its smaller exit pupil. The 10 x 50 though can be hard to hold for some and may require a tripod due to its 10x magnification. Even the 7 x 50 view improves with the use of a tripod. There are several tripod setups on the market, which allow steady viewing, overhead viewing, and easy scanning of the sky without neck cramps. In fact, for certain types of viewing like variable star, comet, and asteroid observing, binoculars on a tripod is essential to give a steady view of the field, take notes, and use star charts. Also more detail can be seen when observing the moon or star clusters.

The recommended size binoculars for observing different objects are as follows:

AuthorCometsVariable StarsNova
NorthAt least 16x for any aperture7 x 50 , 10 x 507 x 50, 8 x 50
LillerLarge aperture 7 x 50 , 10 x 50, 11 x 80
MuirdenLarge aperture8 x 35, 20 x 8010 x 50, 15 x 80
Martinez20 x 807 x 50, 10 x50, 12+ x 8011 x 80

Using Binoculars #

Setting the Interpupillary Distance  #

The interpupillary distance is the distance between the centers of each of your eyes. This is usually two inches. To set the interpupillary distance scale go to the small scale from 60 to 70 with a few small divisions located between the eyepieces on top of the bridge of the binoculars. This is the interpupuillary distance scale measured in millimeters. Once measured you may write it down somewhere to remember it or like professional binocular users mark it with pocket knife or a dot of white India ink.

Focusing a U.S. Military Specification Design Binocular #

Turn each eyepiece diopter out to the extreme plus position; that is, screw each eyepiece diopter all the way out from the binocular. Pick an object out to infinity and focus from the plus to the minus position; that is, focus from the extreme out plus position inward.
The image at first will be out of focus as you move the eyepiece inward. It will slowly come into focus. When it is sharp, Stop! Do not focus back and forth. This is not the correct way to focus an instrument, whether a telescope or a binocular. Do this for each eyepiece diopter.

Focusing a “Center Focus” Binocular #

Select a distant object. Turn the center focus wheel counterclockwise until the eyepieces are all the way out. This is the extreme “plus” position. Now close your right eye or cover the right objective with your hand and slowly focus inward for the left eye until the image is sharp. Stop! Do not focus back and forth. If you do, you will have to start over. Turn the right diopter eyepiece out to the extreme plus position and now close the left eye or cover the left objective with your hand. Now slowly focus inward for the right eye until the image is sharp. Stop! Focusing errors will result if you do, and you will have to start over.

Holding the Binocular #

First, to use your binoculars put the neck strap around your neck. Then to hold a pair of standard size binoculars like the 7 x 50 or the 10 x 50, you simply use both hands and put one hand around each barrel of the binocular and put them to your eyes and observe.

For giant binoculars or even the high magnification 10 x 50 you can use one of the following techniques:

  1. Grab the end of the barrels and let them rest back into your eye sockets. Be careful. This could be painful.
  2. Hold the end of the left barrel with your right hand and let the right barrel rest on your right forearm.
  3. Prop yourself against a building, wall, or tree.

Bibliography and Comment #

Bishop, Roy. “Binoculars.” Observer’s Handbook 2007 (2007): 54-57. Thorough, but technical article on practically every facet of the binocular, whether standard size or giant.

Crossen, Craig and Tirion, Wil. Binocular Astronomy. Richmond, Virginia: Willmann-Bell, Inc., 1992. Intermediate to advanced. Comes with The Bright Star Atlas 2000.0. Full of information on star fields and has tables for star clusters, nebula, variable stars, etc.

Dickinson, Terence and Dyer, Alan. The Backyard Astronomer’s Guide. Buffalo: Firefly Books (U.S.) Inc., 2002. Good review of binoculars, not to mention telescopes and accessories. All photographs in this book are by amateurs.

Harrington, Philip S. Star Ware The Amateur Astronomers Ultimate Guide To Choosing, Buying, and Using Telescopes and Accessories. New York: John Wiley & Sons, Inc. 2007. Good review of practically allastronomical products and the companies who make them. Addresses and phone numbers are provided.

Liller, William. The Cambridge Guide To Astronomical Discovery. Cambridge: Cambridge University Press, 1992. Good review of binoculars and well-known amateurs who use them in their work. Intermediate to advanced. This book covers mainly, comets, variable stars, and nova.

MacRobert, Alan M. “A Pupil Primer for Stargazers.” Sky & Telescope Magazine, May, 1992: 502-504. Excellent, but technical article on the exit pupil, eye relief, and magnification.

Martinez, Patrick, Editor. The Observer’s Guide to Astronomy, Volumes 1 and 2. Cambridge: Cambridge University Press, 1994. Although these books cover binocular astronomy in terms of nova, variable stars, and comets, the material tends to be technical, advanced, and traditional.

Moore, Patrick. Exploring the Night Sky With Binoculars. 4th Edition. Cambridge: Cambridge University Press, 2000. This is a thoroughly written introductory book for binocular observers.

Muirden, James. Sky Watcher’s Handbook. New York: W.H. Freeman and Company Limited, 1993. Basically, a good review of the use of binoculars and telescopes for different types of observing. Intermediate to advanced. There is always something interesting or important to read or refer to in this book. An amateur who has specialized in observing a particular object writes each area of observing.

North, Gerald. Advanced Amateur Astronomy. Cambridge: Cambridge University Press, 1997. Basic review of binoculars. Other topics well covered, especially, lunar and planetary material. Book is advanced and technical, but full of useful information.

Peltier, Leslie C. The Binocular Stargazer A Beginner’s Guide to Exploring the Sky. Waukesha, Wisconsin: Kalmbach Publishing Co., 1995. One of the best introductory books to binocular observing written by one of the great amateur observers. Tells you what to observe and how to find it.

SeyFried, J.W. Choosing, Using & Repairing Binoculars. Ann Arbor: University Optics Inc., 1995. Good review of binoculars covering practically every issue concerning basic optics, cleaning and care, mechanical design, manufacturing, and repair, but lacked information on astronomical observing.