Binocular basics explained

One important, though little heralded, piece of equipment for the voyager is a good pair of binoculars. The inexperienced binocular shopper may find himself swimming in a sea of foreign terms and acronyms. Choosing a pair that fits your needs involves understanding the nomenclature used to describe binoculars.

Magnification is the degree to which an image is increased in apparent size. The unaided eye produces a “life size” image representing a power of one. By the same token, a 7-power binocular yields an image seven times larger than that received with the naked eye. Magnification is represented as part of a series of numerals, such as 7×50 or perhaps 8×40. Such a designation is traditionally pronounced as “7 by 50” or “8 by 40.” The second number represents the size of the objective lens in millimeters. So, with a number such as 7×50, we can determine both the magnification and light grasp.

Light grasp refers to the amount of light that can enter the instrument at any instant in time. While what happens to the light once inside is determined by many factors, light grasp is determined solely by the surface area of the objective lens. One misconception concerning the light grasp of binoculars is that an instrument’s light grasp is the sum of the photons striking both objectives at any instant in time. This is not so. While the ability to use both eyes enhances contrast and resolution (partially through more relaxed viewing), binocular viewing does not increase brightness. Some might argue this point. However, this argument is more at home in labs and with armchair opticians than in a binocular showroom.

Field of view is the width of land or sky that will appear through the binocular. It may be represented as an angular or linear measurement. A typical 7×50 binocular will provide a field of about 7.1° to 7.5°. This will often be represented as so many feet at 1,000 yards. For example, if a binocular had a field of view of 300 feet at 1,000, you would be able to view the length of a football field at a distance of a little more than half a mile.

Exit pupil is the size of the light beam emitted from the eyepiece. Exit pupils may be measured directly or may be determined by dividing the magnification into the diameter of the aperture in millimeters. A large exit pupil (up to about 7.5 mm) is especially useful in astronomy because it enhances image brightness.

Eye relief is the distance the eye can be from the eyepiece and still see the full field of view. Long eye relief is especially useful for people who must wear glasses while observing. It should be noted that individuals who wear glasses to correct for being near- or far-sighted, but who do not suffer from astigmatism, may observe perfectly well without the aid of glasses. When you focus a binocular or other optical instrument, you are really merely compensating for dioptric strength of your eyes.

Close focus is the closest point at which the binocular can be sharply focused. Some instruments can focus on objects at less than five feet; normally one can expect close focus to fall within the 10- to 20-foot range.

Three terms explained Three terms have cropped up over the last few years that are being used in binocular comparisons: relative brightness index (RBI), relative light efficiency (RLE), and twilight factor.

Relative brightness index is calculated by simply squaring the exit pupil, which can in turn be determined by dividing the magnification into the diameter of the objective lens in millimeters. For example, a 7×50 binocular has an exit pupil of 7.14 mm, which, when squared, will offer a RB of 51.

Relative light efficiency is used in an effort to compare instruments with higher-quality optical glasses and optical coatings to instruments of lower index glass and less efficient coatings. RLE is calculated by taking the RBI and adding 50%. That means that our 7×50 binocular, which has an RBI of 51, would have a relative light efficiency of 76.5.

Twilight factor can be used to compare the image clarity between two instruments during twilight or other low-light conditions, and it may be expressed as the square root of the aperture in millimeters multiplied by the magnification. Thus a 7×50 binocular would have a twilight factor of 18.7

Now that a brief description of RBI, RLE, and twilight factor has been presented, I feel compelled to say that I have offered this information so the consumer will understand the language that might by used in sales literature or a sales pitch. I cannot think of a situation in which I would use any of the three measurements to make a buying decision. Why not? First of all, while these calculations are straightforward, what they represent is often misunderstood and ambiguous, to say the least. For example, the exit pupils for 7×35, 8×40, and 10×50 are all 5 mm. This means that they all have an RBI rating of 25. However, the 50-mm instrument has a much greater light grasp and clarity despite the increase in magnification. And, finally, these calculations presuppose equal quality of material, design, and production. The stats for the best 7×50 are going to be the same as for any low-end import of the same aperture and magnification.

Anti-reflective coatings Few features in binocular ads are touted more than the quality of anti-reflective optical coatings. Some companies try to illustrate the benefits of their coatings by showing a photograph, half of which is crystal clear and the other half looking like it was shot through a piece of frosted glass. This is not really a fair comparison. Some of the photographs show a much greater difference in clarity than one would find when comparing a non-coated lens system to the finest multi-coated system available, let alone in comparing two similar coatings. The bottom line? Multi-coated optics increase light transmission by as much as 14%. However, comparing the multi-coatings of two manufacturers is difficult to do and isn’t much help when choosing a pair of binoculars.

One crude way to perform an in-store test of the anti-reflective coatings of instruments you are considering would be to place your hand over the ocular and look down at the objective lens as if you were looking for dust. In an instrument with the standard magnesium fluoride coatings, you will probably be able to see your face quite well. In other instruments featuring multi-coatings, you will see only a very dim image, illustrating that more of the light is going through the instrument and is not being reflected back out into space.

You can’t tell the quality of a coating by its color. Case in point: In the 1960s, binoculars with objectives having deep-blue tints became quite popular, and many supposed they were seeing better through optics with such coatings. Many of these beautiful coatings were by-products of magnesium fluoride being deposited at a temperature far short of the ideal as a production shortcut.

Some have also speculated that the coatings are to “protect” the glass surfaces. This, too, is an erroneous assumption. Magnesium fluoride, at 575 on the Knoop hardness scale vs. 520 for BK7 glass, is harder than the glass beneath. But, at a thickness of 0.000004 inch, it offers little protection. Contrary to what many binocular users believe, power is one of the least attractive and least important features in an instrument. Whether using a telescope or binocular, one should not use more “power” or magnification than is necessary to do the particular job it was selected for. When you increase magnification you:· decrease image brightness by spreading available light over a greater area· decrease the field of view, making objects harder to find and keep centered· introduce more vibrations· accentuate atmospheric disturbances· accentuate imperfections in the optical elements

As with any rule, this too has an exception. Instruments with higher magnifications tend to provide more contrast when used for astronomy. And, considering that binoculars provide a very wide field of view to start with, higher power, say to 10x in a 50 mm model or 20x in an 80 mm model, might be a good trade-off of field size versus apparent image brightness.

The most common reason binoculars are brought to us for repair is that they are out of collimation and the user is seeing a double image. The second most common reason is to have moisture, dust, or fungus removed from the prisms and lenses. To “just clean” a binocular, as the customer usually requests, can cost up to four times as much as a thorough collimation alone! Why?

The average collimation job requires that an objective lens and/or at least one prism be re-centered on the optical axis. With some models this can be accomplished without even going inside the instrument. However, cleaning the instrument properly requires that all the optical elements (with the possible exception of the ocular system) be removed. Then, each prism must carefully be re-seated on the prism shelf at a perfect 90° angle to the facing prism, and they must then be re-strapped, glued, or both. Only after this process has been performed for each telescope is the instrument even ready to begin the standard collimation procedure. All this time, the repair meter is ticking away, and the more expensive waterproof, nitrogen-filled instrument begins to look better and better. It might be good to mention that most of the moisture damage does not come from the binocular being splashed, dunked, or rained on; it comes from rapid changes in the internal temperature of the instrument resulting in moisture condensation.

Prisms examined You may find certain low-end binoculars advertised as “prismatic binoculars,” as if this were a brand-new concept. It means absolutely nothing to the consumer. With the exception of a very few instruments still employing mirrors of polished metal (I haven’t seen one of these in many years), all binoculars are “prismatic.” If they were not, they would more correctly be called “field glasses” or “opera glasses.”

It is becoming very common to hear that roof-prism binoculars are better than Porro-prism models and that they are the way of the future. Well, while roof-prism instruments are becoming the way of the future, they are, all things considered, not superior to their larger Porro-prism cousins. While the roof prism configuration allows binoculars to be smaller and lighter, making them more convenient to use, the proximity of telescopes causes a loss in depth perception, and such a prism configuration comes with its own set of drawbacks. A roof-prism splits the image into two equal parts and then re-combines them by two separate total-internal reflecting surfaces. This process, along with the wave nature of light, causes a “phase shifting” of the light that leads to a loss of contrast. Some manufacturers are now coating the prisms with anti-phase-shifting coatings to increase contrast and close the quality gap between roof-prism and Porro-prism models. However, phase shifting is only part of the problem. Roof-prism designs are considerably more complex than Porro-prism models, and the cost to manufacture and test a roof prism is considerably more.

This quick tour of binocular issues highlights their complexity once you delve into the optics. Luckily, voyagers need not get so technical. The best approach is to read the specs, check your budget, and then consult with a reputable dealer of optical equipment.

By Ocean Navigator