Binoculars are one of the most difficult pieces of equipment to make a decision on. With the large variety of style, types, brand and uses for binoculars, making this decision sometime can be confusing if some basic terminology is not understood or the specification numbers look foreign. To narrow down the large number of binoculars out there, you need to consider first what your main use will be. Will you be using them for bird watching on the feeder in the back yard, spotting game a long distance off or during bow hunting at the end of the day? Below is a list of terms that better explain the parts of the binoculars. We will go into depth a little further a little later.
- Objective Lens
- Exit Pupil
- Eye Relief
- Field of View
- Eye Relief
- Lens Coatings
Binoculars are commonly described by a set of numbers such as “10×25” or “10×42”. When trying to decipher these two numbers, the first number represents the magnification or power of the binoculars. In our example, a 10×42 binocular has a power or magnification of 10 x, which magnifies the image 10 times, closes than if you were looking at it with the naked eye. Typically most binoculars come with a fixed magnification, but there are some binoculars out there that have a zoom power and these will be displaying a number like 10-22×50. This example the image can be zoomed in from 10 times closer to 22 times closer. With zoom binoculars, the exit pupil (which will be explained later) will decrease as the magnification is increased. Two major consideration when trying to determine the magnification of the binoculars you would be purchasing is the larger the power the harder it is to hold the binoculars steady and when looking at optically equal binoculars, the larger the power the darker the image will be. This is typically why many hunters or bird watchers will choice a compromise power such as 7x or 8x.
When looking at the size of the binoculars, the second number is referred to as the objective size. In our example, the 10×42 binoculars will have an objective size of 42, which means the lens measures 42 millimeters in diameter. In optically equal binoculars, the larger the objective lens the brighter the image will be during low light situations due to the larger diameter lens allowing more light to entering the binoculars and be sent to your eyes. One negative to a larger objective lens is the larger the lens typically the heavier the binoculars are.
Binoculars are manufactured in two configurations, the roof prism or the porro prism style. Roof prism binoculars are manufactured with the prism being in a straight line within the optical tubes making for a little more compact binocular. This type of binoculars is harder to manufacturer due to the precision needed to make sure the images are a line in one plan, resulting in a more expensive binocular. Porro prism binoculars can be identified by their offset tubes. The objective lenses are not in a straight line like the roof prism style. In today’s binoculars, the technological advances have allowed the quality of the image in a roof prism binocular to be equal of that of a porro prism binocular. This is not necessarily the case when looking at binoculars in the same price range. A cheaper priced porro prism binoculars will have the same image quality as the mid price range roof prism style. To get an outstanding image in a roof prism binocular, you will need to look at the high end roof prism models.
When looking at the two different styles of binoculars, not all roof prism or porro prisms are equal in quality. The two basic prisms are the BaK-4 and the BK-7. The BaK-4 prisms are the best and produce a quality image. The BK-7 prisms are usually put in low price binoculars.
Exit pupil is a measurement of light that is transmitted through the binoculars to your eye. This calculation is done by taking the objective lens and dividing it by the power or magnification. For example, you have a pair of binoculars that are 10×42; you take 42 divided by 10 which gives you an exit pupil of 4.2mm of light. Now let’s take into consideration a pair of 10×25 compact binoculars. If you use the same calculations at the above example, you will see the exit pupil is 2.5mm, substantially less then the 10×42’s.
Why is this number important? When the light begins to fade at the end of the day, the higher the exit pupil the longer you can see through the binoculars. Now a healthy human eye pupil can only dilate to about 7mm’s, so any measurement above 7mm’s is actually not accepted by the eye and is just wasted. As a person gets older, the eye’s ability to adapt to low light situations decrease, effecting the amount of exit pupil light that it can accept.
Field of View
Field of view is the side-to-side measurement while looking through the binoculars. Typically this measurement is defined in feet or meters at a specific distance usually at 1000 yards or meters. When looking at the field of view (FOV) specifications, there are two things that will determine the FOV distance; the magnification and the eyepiece design. As the magnification increases the field of view will decrease. Example, a 10×42 binocular will have a smaller field of view compared to the same binocular brand in an 8×42. The second determination is the eyepiece design. Wide-angel eyepieces feature a wide field of view and are better for activities that motion is involved like sports or car racing.
Eye relief is the distance between the eyepiece and your eyes while still providing a full field of view. Most manufactures product binoculars with a twist out eye cup which provides eye relief for both the person who wears glasses and people who do not. Long eye relief helps reduce eyestrain.
To minimize the reflection of light on the lens surfaces, anti-reflective coatings are applied to both sides of a lens. There are two main types of coating: a single layer and multicoated. A binoculars with a multicoated lens will greatly reduce the amount of light reflected which increases the transmission of light through the lens. The higher quality of optics, the more lens coatings that are applied, providing higher light transmission, sharper image and better contrast.
You can also read about choosing a riflescope!
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