choosing binoculars

[HenryC]

This is a continuation of my other thread "consider the monocular"

(Originally appeared in Florida Wildlife Magazine, Mar-Apr, 2010).

One of the most important pieces of equipment you can have on your boat is a good pair of binoculars.  They are an essential piece of safety gear useful for navigation, recreation, nature study, and even come in handy when you’re nowhere near the water.  Underway,  almost any kind of optical aid is better than none, but it still makes sense to optimize your optical equipment for the application, while keeping in mind that you will probably want to use them ashore as well, for  sporting events, bird watching, or even stargazing.

A binocular (or “night glass”) is two small  handheld  telescopes mounted together on a frame so that they allow you to use both your eyes at once.  All else being equal, you can see much more with binoculars than with an equivalent size telescope; the brain will fuse the images from both eyes into one and the image will look sharper, clearer and brighter than if you were using just one of them. Their disadvantage is that they are bulkier, heavier, more fragile, and more expensive than a telescope of  the equivalent optical characteristics.  They also have to be matched so that their alignment and focus is set up for your eyes, otherwise your brain will ignore the view from one side and just take in the other and you will get eyestrain and tire.  This is important, there are circumstances on a boat where you may be using your glasses for a long time, perhaps not continuously, but still long enough so fatigue can be a problem. 

The focusing mechanism for boating binoculars MUST allow each eye to be individually focused. As it turns out, for most of us, each of our eyes is slightly different, and unless each half of the glass is tuned to the appropriate eye you will always find them uncomfortable.  You may also find that your eyes change their focus point under different light conditions, fatigue level and over time. In addition, there may be others sharing the binoculars with you and they will have to adjust the focus when it is their turn to look.

The distance between the eyes of your shipmates will probably not be the same as yours.  Binoculars solve this problem by having a hinged frame so you can adjust the inter-ocular distance quickly, and it is a mark of the mechanical quality of the glass that this hinge operates without altering the focus of either side of the instrument relative to the other.  It is also essential that the two telescopes remain parallel and aligned when you use the hinge to bring them closer or further apart. Cheap glasses won’t do this.

There are three types of focusing mechanisms.  The least expensive, (and unacceptable!) focuses both sides with one adjustment knob between the two telescopes. You cannot adjust both eyes separately with this style. Another type uses individual focus on each eyepiece.  This allows a simpler, cheaper-to-build yet structurally stronger design.  Although this is fine for specialized tripod-mounted binoculars which can be adjusted at leisure, it is not suitable for marine use where they may have to be brought into service in a hurry, using only one hand.  The ideal focus control for marine glasses is one that has a central focus knob that focuses both eyepieces simultaneously, and one of the eyepieces (usually the right one) that focuses separately, by twisting it. 

Always focus your glasses on distant objects.  I like to use the horizon, or the edge of a cloud during the day, or a distant boat or landmark on shore.  At night, focus on a star or faraway light. If it is totally dark out, you may not be able to focus at all.  You can always use some illuminated object near you,  but keep in mind that a sharp image of a nearby object will not mean a sharp image on a distant one. Focusing is done by closing the right eye and opening the left,  then sharpening up the left image with the center dual-focus knob.  You then close the left eye, open the right,  and adjust the right eyepiece by twisting it.  There is usually a scale of numbers with a pointer there so you can remember and dial in your personal setting next time you use the glasses. 

It may be necessary to repeat this procedure several times until you are satisfied with the view. During the course of time, or for different conditions, you may need to change focus slightly, but this will usually only require the center knob.  If you take your time and do this right, the view will be sharp and clear, and there will be no fatigue of strain, even after hours of continuous lookout duty.  Incidentally, actually locating objects in glasses can be tricky; it is hard to place the field of the glasses on distant object, especially if the field is cluttered with many distractions (like attempting to read a distant buoy number against the background of a busy shoreline).  With the glasses up to your face, you lose the overall layout of the scene and wind up pointing them everywhere except at your target. Here’s a tip: look directly at the object you wish to examine, then, without breaking eye contact with it, bring the glasses up to your eyes.

The hinge and focusing arrangement is a good indicator of the overall quality of your glass. It should be easy to use, smooth, and the center focus knob should operate so both eyepieces move together uniformly without losing  focus relative to one another.  There should also be no excess grease squeezed out of the mechanism, a clue that the manufacturer cut corners and compensated by over-lubricating.

If you wear spectacles, you may be able to just remove them when using your binoculars if you are near- or far-sighted.   The focusing adjustment will take over for your spectacles.  However, if you suffer from astigmatism, you may have to keep your specs on while using the binoculars.  If this is the case you may have to buy binoculars with  additional  “eye relief” (the required distance between the eyepiece and eyeball, measured in millimeters) so your spectacles will fit between your eye and the eye lens. This will add to the cost.  A moderate amount of eye relief is good even if you don’t wear spectacles (it is more comfortable to not have the eyepieces right up against your pupils, and it keeps the salty, greasy moisture from your eyes and face from getting on the lenses and damaging the lens coatings. Quality binoculars will publish the eye relief in the instruction manual and in their advertising copy.  If you are not astigmatic, high eye relief is not critical.

The purpose of marine binoculars is not to make distant objects look bigger, it is to make dark objects and dim scenes appear brighter.  This is why they are called “night glasses”.  During the daytime, there is plenty of illumination and the faint light performance of the glass is not an issue.  But at night, dim light capability is critical.  Assessing this important parameter is done by noting the numbers printed on the binocular frame.  Most common binoculars are “7x50", or “7x35", but there are many other common ratings as well, such as “6x30", “10x50", or even “11x80". 

The first number is the magnification; how many times bigger (or closer) an object will appear through the glass. The second number is the size of each front lens in millimeters.  So a 7x50 binocular (the most common and probably the best overall night glass) has a magnification power of 7 and a 50 millimeter diameter objective lens (about two inches). These two numbers give you a very good idea of the performance of the glass.  Dividing the objective lens size by the magnification gives the “exit pupil”.  So in a typical 7x50 night glass the exit pupil is about 7 millimeters.  The exit pupil is the size of the cone of light coming out of the eyepiece where it enters the pupil of your eye.  In the classic 7x50 night glass, the light cone just  fits into the 7mm pupil aperture of a healthy, young, dark-adapted human eye.  If the exit pupil were any larger, the cone wouldn’t fit into the eye and some of the light would be wasted, the image would be too dim.  If the exit pupil is less than 7mm, the magnification is too great for the dim light conditions; whatever you are looking at is spread too wide  and the image is bigger, but fainter.  A lower magnification also gives a wider field of view so you see more of the scene through the glass, making it easier to find things and track them from a rolling deck. Since the purpose of a night glass is to allow you to see in the dark, and higher magnifications are hard to use on a platform that is rocking, the 7x50 is considered the optimum night glass by most authorities.  During the day, of course, your eyes are not dark-adapted and a smaller exit pupil can be tolerated. For an older boater (like myself!), who has trouble keeping his hands steady and whose pupils no longer dark-adapt to a full 7mm, a lighter and more manageable 6x35 would work just fine.

There are many other considerations when buying binoculars.  Should you spend extra for waterproofing, extra-ruggedized construction, corrosion-proof finishes or super anti-reflection and anti-glare coatings on the lenses?  If you can afford it, exotic materials can give you superior strength for less weight. Should you get binoculars with a built-in, illuminated compass, so the bearing to the object you are looking at appears through the glass?  There are different types of construction and focusing mechanisms, and even high-tech night-vision devices. Some boaters prefer extra wide angle views, or extra eye relief.  And now there are image-stabilized binoculars which use an internal mechanism to greatly reduce the shake and quiver from the image. Some of these features may be right for you, but I would suggest you start off with a basic but quality 7x50 with the focusing arrangements I described until you become more familiar with how you are going to use your glasses and learn what to look for as far as performance is concerned. Specialized marine binoculars from name brands are very expensive, a real treat for an old salt, but perhaps too much for a beginner.  Any good pair of glasses,  well taken care of,  will stand up to a lifetime of marine use and will always work as a reliable backup when you decide to upgrade.  They will also be fine for non-boating use; a 7x50 night glass is perfect for astronomy and good for sports and nature study, although a bit bulky for daytime use. Remember, during the day, you don’t need those big  lenses to scoop up enough light, especially when your eyes aren’t dark- adapted anyway.

How much will a decent night glass set you back?  If you look carefully and get good advice, you can pick up something quite acceptable from a reputable house for between two and three hundred dollars, but a top-of-the-line item might cost you several times as much. I shop at amateur astronomy mail order outfits, their customers are very  knowledgeable and they have high-volume sales, (which keeps prices competitive). They also have good technical advice from knowledgeable service representatives available over the phone. In my experience, they will recommend something right for you.

[Shawn]

"Another type uses individual focus on each eyepiece.  This allows a simpler, cheaper-to-build yet structurally stronger design.  Although this is fine for specialized tripod-mounted binoculars which can be adjusted at leisure, it is not suitable for marine use where they may have to be brought into service in a hurry, using only one hand.  The ideal focus control for marine glasses is one that has a central focus knob that focuses both eyepieces simultaneously, and one of the eyepieces (usually the right one) that focuses separately, by twisting it.  "

I think you misunderstand how the individual eye focusing binoculars are used. You focus each eye *once*  and then you never have to touch focus again. Doesn't matter if you change what you are looking at. Everything is in focus from minimum focus distance out to infinity. For marine use where you need to quickly spot something one handed these are the easiest/quickest to use.

Look at every marine Steiner binocular Defender carries. They are all individual focus(Steiner calls it "Sports Auto Focus"), not central focus. Ditto all the marine Fujinon's. It is the cheaper models that get away from this and move down to center focus, not the other way around.

Shawn

[HenryC]

I have to continually refocus binoculars while I'm using them becasue my eyes focus differently under different lighting conditions.  Only in astronomy, when the eye is perfectly dark adapted, can I get the focus just right and keep it for long periods of time.  In a marine environment, when I am looking at the chart, or catching the glare of the running lights, fumbling with two focussing knobs is just too much of a hassle.  Now if I were a lookout on a large ship, on a darkened signal bridge for long periods of time in complete darkness, the dual focus would work for me. 

For me, the ideal binocular setup up is a center focus knob with an individual focusser on one eye.  I still have to focus frequently due to illumination or fatigue, but I only need the center focus, once I correct for my left-right bias.  Even so, if I take my away from the glass to do something, like look at the compass, I have to refocus both when I use the glass. 

Maybe its just the way my eyes work, or maybe I'm really picky about keeping the focus perfect.  But I know I have to refocus frequently, even when I'm using a monocular.

[Shawn]

When I focus mine I don't need to touch them again. I am also very picky about focus being heavily involved in photography for many years. In photographic terms the binoculars I'm talking about are really fixed focus set to their hyperfocal distance and each eye piece has an individual diopter adjustment.

Shawn

[HenryC]

When I focus mine I don't need to touch them again. I am also very picky about focus being heavily involved in photography for many years. In photographic terms the binoculars I'm talking about are really fixed focus set to their hyperfocal distance and each eye piece has an individual diopter adjustment.

Shawn

You're right, since "infinity" for binocular users is not that far away, and rarely do we use binoculars to look at nearby objects, something focussed at infinity should work fine for astronomy, marine use, and most sporting activities.  We should only need to focus to accomodate our own eyes. 

Maybe I have an eye problem, since I can't seem to hold focus under varying lighting conditions.  Fortunately, I do have sharp distance vision, I can tell because of my astronomy work.