Designation

All binoculars are labeled with numbers delineating their size and magnification. These numbers might look like 8x42 (read as 8 by 42). The 8 in this example is the power or magnification. The 42 is the diameter (in millimeters) of the objective lens, the end through which light enters the binoculars (see Figure #1 or Figure #2 in "Basics I - Designs"). These numbers define only magnification and size and not any other aspects of the image or optics quality.

Full-size binoculars generally range from 7 to 12 power. The objective lenses range from about 30 to 50 millimeters in diameter. Of these sizes the most popular are powers from 8-10.5 with 40-50 millimeter objective lenses. The objective lens diameters for scopes are larger and the most popular top out at about 85 millimeters.

Objective Diameter

In the example above, the objective lens diameter is the 42 in the 8x42 designation. The larger the objective diameter, the more light there is gathered to transmit to the eye. This translates to greater potential detail and greater color resolution. Of course, the quality of the optical system, how much magnification there is (generally, more magnification = less light transmitted) and how stable the optic is mounted or held will determine how much can be seen. A poor 50-mm objective lens will not give the detail, brightness and color that a high-quality 42-mm lens will.

Larger objective diameters also mean more glass and weight. In the case of a spotting scope where it is mounted on a stable tripod and weight is not as great of a concern, the larger objectives are helpful. Legitimately, a 50-mm objective lens is about as much as anyone would want to carry in a binocular with today's materials. Because of the weight involved, people will often opt for 42-mm objectives. Top of the line optics are able to deliver extremely sharp views with a 42-mm objective. In theory, a 35-mm objective in normal daylight should be able to deliver all the detail we need at normal birding ranges. When you look into shadow, are in the woods viewing birds at a distance, in failing light at dusk and dawn or any other condition where bright light is compromised, a larger objective helps.

Larger objectives are especially helpful when either the distance increases or bright sunlight is compromised by any of the factors mentioned above. Both image detail and colors are limited when the distance increases with smaller objectives... even in bright light. The basic physics of light are such that the details and colors simply blend together with smaller objectives. To get a sharp view with good color, a larger objective is simply necessary. This effect becomes more apparent as the distance increases or ambient light decreases. Even looking into a shadow, a 40-50-mm objective is necessary to deliver enough light to excite the color receptors in our eyes and to give good detail.

Magnification

Commonly in the past the most popular magnifications for field size binoculars were either 8 or 10 power. This means that the view that is seen is either 8 or 10 times as large as with the normal eye. Figures #5 and #6 below give an idea of the difference in view at these two magnifications.

8X	10X
Figure #5	Figure #6

Several other factors are affected by magnification, most notably depth of field, field of view, image brightness and weight, which impacts our ability to hold a binocular steady. These factors affect how well we can decipher details, how easy the optics are to use and the fatigue using them causes.

As a matter of simple physics, image brightness decreases with higher magnifications (assuming the optic quality and objective diameter remain constant). In other words, as we zoom in on an image, the brightness drops (try it with a zoom lens on a scope!). This will determine how well you can distinguish color and detail.

Depth of field is how much depth stays in focus in the field of view. The higher the magnification, the shallower the region of sharp focus in the image. This affects closer objects more since the depth of field is greater at larger distances. For example, imagine focusing on a bird 15 feet away. Maybe everything in view at 14-17 feet away is still reasonably sharp. If you are looking at a bird 60 feet away, maybe everything from 55-70 feet away is still sharp. The bird at greater distance can move around relatively more while still staying in focus whereas relatively small movements by the nearer bird require focal adjustments to maintain a sharp view. This may cause fatigue with extended field time, not only because of adjusting focus but because our eyes are also doing some of this work (depth of field is discussed in greater detail in Basics III).

Field of view decreases as we, in essence, zoom in on a bird with higher magnification. You can see this difference in figures #5 and #6 above (field of view is discussed in greater detail in Basics III). The effects are that finding a bird and then following it around as it moves can be more difficult. Younger children or beginners often have trouble finding birds in smaller fields of view.

A last consideration involves how steadily you can hold your binocular. Making out details requires being able to hold the optics steady. When you increase magnification more movement is apparent in the view. Especially if you are tired after a long day in the field, holding your binoculars steady can be difficult. A 12-15x binocular (with the exception of image-stabilized optics) is just too difficult to hold steady and requires a tripod.

Several of the manufacturers have now come out with 8.5x and 10.5x binoculars for that extra little "kick" in magnification. This extra little bit in the case of the 8.5x might be a comfortable addition for those who are used to 8x binoculars and the 10.5x just a little bit more for those who are used to 10x binoculars. With children, due to weight and field of view considerations, a 7x compact binocular is easier to hold steady and find things.

Weight

The two most notable concerns with the weight are related to fatigue and the ability to hold steady. A well-balanced heavier binocular will also have more inertia, i.e. resistance to motion. For fatigue related to a having a heavy binocular hanging from your neck, several kinds of binocular straps and harnesses have been designed to alleviate this problem. It might seem reasonable that a 28-oz pair of binoculars is easier to use and hold than a 35-oz pair but sadly it is not so simple. The ease of use is much more related to how well balanced the optics are, the distribution of weight into your arms and other factors that will be discussed further in this and subsequent articles.

Most of the popular full-size binoculars range from about 25-40 oz. Weight may be the single reason people most often opt for 42-mm objective lens. Optics with 42-mm objectives generally range from 25-32 oz while 50-mm objectives range from 28-40 oz. Weight is a major consideration when considering how much fatigue carrying a binocular around in the field all day will cause.

What this doesn't tell us is how well designed or balanced the optics are. A comfortable placement of the focusing knob, the fit in your hands and how evenly the weight is distributed into your arms (balance) may be more important than simply how much the binoculars weigh. Consequently, a poorly designed and balanced 28-oz binocular can cause more fatigue than a 35-oz pair.

Eye Relief and Eye Cups

Modern binoculars are almost always designed with eyeglass wearers considered. Eye relief and the design of the eyecups might be of most importance to eyeglass wearers since their eyes are physically further away from the eyepiece while wearing glasses, and being able to adjust the binocular to suit their own glasses is important.

For many visual conditions, notably near- and far-sightedness, the binocular focal mechanism can compensate for the problem (astigmatism is not corrected by the optic focus). Most of the time, we look around with our eyes and then use the binoculars to look closely at something. If you wear glasses, this means you must take your glasses off or adjust the eyecups so that you can look through the binoculars with your glasses on.

Eye relief is defined as the distance from your pupil (where the image focuses) to the surface of the optic eyepiece. To be precise, the image coming out the eyepiece is actually focused behind the face of the eyepiece. For comfort this distance should be at least 10 mm. Most wide-angle eyepieces have a shorter eye relief distance than this. Getting your eye physically closer than 10 mm will cause undue eye fatigue not to mention physical fatigue fighting off your automatic blink response.

Eyeglass wearers need greater eye relief to compensate for the distance their glasses stand away from their pupils. This is generally about 12-20 mm where you can see about 80% of the field at the low end and the whole view at the high end. Binocular eyecups come in multiple designs to provide longer eye relief. Standard rubber eyecups can be folded back to allow the lens of glasses to sit physically closer to the eyepiece of the optic. Some of the newer and high-end optics have adjustable eyecups so the distance can be preset for comfortable viewing while wearing eyeglasses.

For those with severe eye problems requiring larger or thicker glasses, “High Eye Point” binoculars are also available. These are designed with greater eye relief than 20 mm but they are generally not recommended. The problem with these are that they can be difficult to keep the circular area that is projected from the eyepiece both centered and in focus over your pupil. They also tend to "black out" unpredictably when the eyecup is collapsed. Consequently, it might be better to use optics with a standard eye relief, and not be able to quite get close enough to see the whole field.

Exit Pupil

While discussing what comes out of the eyepiece, we should consider the exit pupil. The exit pupil is the circular beam of light that comes out of the eyepiece of the optics. If you hold your binoculars at arms length and look at the eyepiece, you will see a bright circle of light on the eyepiece. The diameter of that circle of light is the exit pupil. It is calculated by taking the objective diameter and dividing it by the power of the optic. For instance, a 10 X 50-mm pair of binoculars has a 50 mm ÷ 10 = 5 mm exit pupil.

The exit pupil has often been used as a measure of how bright a binocular is, under the premise that the larger the exit pupil, the more light there is coming out of the eyepiece. This is absolutely NOT true. Under this premise a 7 X 35-mm optic delivers as bright an image as a 10 X 50-mm optic, since both 35mm ÷ 7 and 50mm ÷ 10 equal 5mm. Given that the quality of optics is the same, the 10 X 50mm binocular is much brighter.

The exit pupil measurement does have some use though. In bright light, your eye pupils are normally open to about 2-3 mm. If the exit pupil of the binoculars is close to this number your eye must be directly centered over it in order to see through the optics. This can be important if you are on a bouncing boat where keeping your eye centered exactly over the exit pupil might be difficult.

Some articles claim that optics with larger exit pupils than your eye pupils are a waste of light. If the exit pupil was used as a measure of brightness this may be true, but only because brightness does not increase when the exit pupil exceeds the diameter of your eye pupil. This is a key issue with compact binoculars. A pair of 10x28 or 8x21 compacts have an exit pupil that is less than 3 mm which can be especially difficult to center exactly over the pupils, and thus difficult to see properly with. This is further complicated in dim light when your eyes dilate. By about the age of 40, our eyes reach a maximum dilation of about 5 mm, so if you're using binoculars in all light conditions you would want at least a 5 mm exit pupil.

Exit pupil may indicate the ease of use but has no real value in reference to brightness. This measurement fails to take into account optical glass quality and alignment of optical elements. These factors have a greater impact on how bright an image is delivered to your eyes.

Twilight Factor

While on the topic, we should also mention the myth of twilight factor. This is another measurement based on the physical size of the optic and does not take optic quality into consideration. The twilight factor is derived by multiplying the objective diameter by the magnification and then taking the square root of this product. Thus a 10x40mm binocular has a product of 400 as you multiply the objective diameter by the magnification (10 X 40 = 400). The square root of this is 20 (√400=20).

This number may seem reasonable when comparing a 10x42 pair of binoculars that has a twilight factor of 20.5 with an 8x35 pair that has a twilight factor of 16.7. In reality, it has little validity since it is actually objective diameter that might make the 10x42 brighter than the 8x35 optic. This is immediately apparent if you compare an 8x42 with a twilight factor of 18.3 and the 10x42 with a twilight factor of 20.5. As noted before, brightness declines as magnification increases. Again, this does not take optic quality into consideration that has tremendous impact on the perceived brightness of the optics.

Conclusions

• The optic designation only defines size and magnification - not optical quality.
• Greater objective diameters are required for optimal viewing in all conditions.
  
• There are "trade offs" that come with higher magnifications.
• Weight is an important issue but ergonomics may be more critical.
• Eye relief is important to all users. The eyecups adjust to allow eye glass wearers the ability to look through their binoculars without always having to first remove their glasses.
• Neither exit pupil nor twilight factor tells you anything about actual performance or optic brightness.

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