Basic Binocular Knowledge for the Hunter and Bushcrafter

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Binocular Basics - Optics Basic Binocular Knowledge for the Hunter and Bushcrafter

Bushcraft and hunting can both benefit from good optics. The rifle scope is a hunters first priority, but good binoculars are a close second. One safety rule in hunting is never to point your rifle at a target you have not identified. That limits the use of a scope to known game targets. A good pair of binoculars is therefore essential for _finding_ and identifying game safely.

A Bushcrafter can also benefit greatly from a good pair of binoculars. They allow checking for trails and portages, watching animals, birds, flowers and even insects from a distance. They also allow for spotting other people a long way off, and determining what they are doing.

You might have noticed the number of times I stressed a "good pair" of binoculars just now. There is a reason for that. Quality binoculars allow for use for extended periods without eye strain, headaches, and blurred vision. Poor binoculars cause these same maladies.

Now, I did not say expensive , or cheap; I referred to good quality and poor quality. The difference is only partly reflected in price. There are several things that make a difference in binocular quality. The quality of the lens is the first factor, and it influences the clarity of the image, whether the edges of the field of view are distorted from spherical aberration, whether the colors of the image are distorted from chromatic aberration, and also to a degree how well light is transmitted (image brightness.)

True binoculars also have prisms inside, to shorten the length while allowing a long optical path. The prisms need to be of the same quality as the lenses.

The next major factor will be the lens and prism coatings. If you look at the lens from the side, it will have a color-deep purple, blue, green, or even reddish. This is caused by chemical coatings on the optics. The coating helps to cut down on light reflection at the interfaces between lenses, prisms, and the atmosphere. Uncoated lenses will lose a lot of the light inside the binocular, causing an image that is noticeably less bright. There are several different chemical coating systems, and all have their claims. What is important to us is how many lens surfaces are coated and how well they are coated. The common terms used are “Coated Lenses”; Fully Coated” ; “Multi-Coated” ; and Fully Multi-Coated”. The “Fully Multi-Coated” is the best, and that is what you should look for.

In addition to the optics and their coatings, a major element is the mechanical system that holds the lenses. The two barrels of the binocular must be very closely aligned in order to give a good image. Poor alignment can be compensated for to a degree by your eyes, but this makes for a lot of eye strain, and soon causes headaches and blurred vision. Now is a time to mention the “Zoom” or Variable Power Binocular. These are almost always going to sacrifice optical quality and precision alignment in order to gain the variable power feature. It is not worth having this feature for serious optical use, and I discourage it when friends ask.

Now that we have covered the basic elements, we arrive at the question of what power and size binocular is best suited for our purpose. Most binoculars are described by two numbers, with an "X" in between. Examples are 6X30, 8X50 , 10X20 , etc.

The first number, and the "X" denote the actual magnification, so a 6X magnifies the image to appear 6 times larger, a 10X is ten times larger, etc. Many people make the mistake of going for the largest magnification they can get, and that is often a poor choice. For most of us, anything over 8X to 10X magnification will give a blurred image when hand held. They need a tripod mount, or at least a steady rest in order to give a solid image. So, you need to decide just how much magnification you need. A birder watching small songbirds might benefit from 8X or 10X magnification, where a hunter looking for deer would be better served by a 7X or 8X. Different uses and different people will have different optimum choices of magnification.

The second number, the one _after_ the "X", is the diameter of the objective lens, in millimeters. So, 25mm (about 1 inch) is much smaller than a 50mm ( about 2 inch diameter). Remember the area of the lens goes up as the _square_ of the radius , so twice the diameter is a lot more than twice the area.

The reason area of the lens matters is that the objective lens is where _all_ the light enters the system. If it is small in area, allowing less light, then the image will be less bright. Large objective lenses gather much more light, and have brighter images.

However, there is a trade off (again), and a larger objective means a larger and heavier pair of binoculars, making carry more of a problem.

The next topic is what makes a pair of binoculars good for low light or night use. This area involves a bit of mathematics, and a few terms not often encountered. We are going to discuss them in steps.

First is "Exit Pupil". This is a measure of the size of the light beam transmitted from the eyepiece to your eye's pupil. It is calculated by dividing the objective lens diameter by the magnification power of the binocular. Example 1: a 6x30 binocular has 30mm. objective size divided by 6X, for a 5 mm. exit pupil. Example 2: a 7x35 binocular has 35mm. objective size divided by 7x, for a 5mm. exit pupil. Example 3. a 10x25 binocular has a 25mm. objective divided by 10x, for a 2.5mm. exit pupil. Example 4. a 7x50 binocular has a 50mm. objective divided by 7 for a 7mm. exit pupil.

So, we have exit pupil sizes from 2.5 mm. to 7mm. What does this mean ?

Well, in daylight, your eye has a pupil diameter of about 3-4mm. so a 2.5mm exit pupil is not getting light to the entire pupil of your eye. A 7mm exit pupil, on the other hand , is wasting light around your eye pupil.

In low light conditions , however, your eye's pupil can expand up to around 7mm. , so that a 7mm exit pupil is exactly right for maximum light transmission.

The key thing here is that if you are only interested in daylight use, a small objective is no real handicap to getting a bright image. The extra size and weight of larger objective lenses is only really useful in deep woods, or at dawn and twilight.

Now that we have defined "Exit Pupil", we can move to another term used to measure how binoculars perform in low light level. This is the term "Relative Brightness", and it is useful merely as a relative measure to quickly compare different binoculars of similar quality.

Relative brightness is the _square_ of the exit pupil. So, a 5mm. exit pupil has a relative brightness of 25, while a 7mm exit pupil has a relative brightness of 50, or about double the smaller exit pupil. This helps to decide just how much advantage you can expect from a larger objective, assuming all other factors are equal.

A different term used for measuring light performance is "Twilight Factor".

The twilight factor is found by multiplying objective lens diameter by magnification, and then taking the square root. A 7x35 binocular would have 7x35= 245, and the square root would be 15.6. A 7x50 binocular would have 7x50= 350, and a square root would be 18.7. A 10x50 binocular would have 10x50= 500, and a square root of 22.3.

Here, the higher magnification has some advantage, in concentrating the available light for your eyes.

After all that, I have to say that these are only useful as rough guides. Lens quality and the coating of the optics will make as much or more difference than the physical limits described above. A high quality 8x30 pocket binocular may give more effective light transmission than a lower quality 10x50 set.

The final issue is what size binoculars are best for different uses. This is at heart a personal decision, but there are certain recognizable task groupings in common use.

For hunting: waterproof, 8x40, 8x42, 10x40, roof prism ; if used at night or in low light levels, then 7X50.

For birding and wildlife watching: 8x40, 8x42, 10x40 roof or porro prism with a close focus of less than 15 feet.

For astronomy , using a tripod: 10x70, 20x80, 25x100.

For Marine (over water use): 7x50 waterproof. For sporting activity: 8x40 or 8x42. For hiking and constant carrying: compacts such as an 8x25, 10x25.

Bushcrafting will tend to find the compact styles of most use, and in fact an 8X20 and a 10X25 are the two most useful sizes for me for every day carry.

When hunting afield, the 7X50 size, though a bit heavy and bulky, serves well in looking into forest gloom, and for spotting game at dawn and dusk. They are also excellent for all around observation use, and give good service when you don't mind carrying the extra weight.

Having stressed that quality is important, I will mention a few brands that are outstanding quality. There are others that are good, and some are inexpensive, but these brands I have owned, and found are consistent in being of good quality: Zeiss, Leitz, Swarovski, Steiner, are my first line brands. Leupold and Nikon are in the slightly less expensive range, and are also of good quality.

If you have found an inexpensive model that serves well, that is a a good choice, but in some of the less expensive brands, quality can vary from one model number to another.

Copyright 1998, and 2011 G Lacy All Rights Reserved