Guide to Binoculars
- Category: numbers
- Category: field of view
- Category: exit pupil
- Category: prisms
- Category: lens coating
- Category: focus
Category: numbers
You might have noticed that binoculars always carry numbers in their name, like Levenhuk Bino Basic 10x40. If you are a beginner you might not know what those number mean. The first number, 10 in our example, is the magnification. In other words, magnification of 10 will make the object 10 times closer to you or 10 times larger than you would see it with a naked eye.
Yet, you have to be careful with this number. 10 is, probably, all you would need for regular purposes. If the number rises, the image becomes more blurred and shaky and less bright. This happens because along with the object, everything else is magnified, including the tremble in your hands. Magnification can be higher than 10, but than you would need a support for the binoculars, for example, a tripod.
The second number, 40 in our example, shows you the diameter of the front lens. The higher the number, the better it is for the image. So, 10x40 is much more comfortable than 10x25, for example. The only disadvantage of the higher number is the increased weight of the binoculars it causes.
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Category: field of view
Field of view is another thing to keep in mind when you are deciding which binoculars to buy. In short, it is the amount of space you see through the binoculars. Say, you have a wall 1000 yards ahead of you. With the field of view 250 you will see 250 feet of that wall without having to turn your head. Logically, with the magnification rising, you will see less of that wall, but in more details. Therefore, the higher the magnification, the smaller is the field of view.
If you wear glasses, you have to consider the eye relief of the binoculars. The eye relief is the maximum distance from the eye to the eyepiece at which the entire field of view is still observable. This distance shouldn’t be smaller than the thickness of your glasses.
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Category: exit pupil
Think of the exit pupil as the pupil of your eye on lens of your binoculars. It is calculated in millimeters by dividing the second number by the first, like this: 40/10=4. The exit pupil can roughly tell you what the brightness of the image will be. For basic purposes 2.5-3 is just fine. For stargazing 5-7 is better. Anything higher is not necessary, except for the cases when the binoculars shake heavily, like on the boat, since 7+ gives a steadier image than any other exit pupil.
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Category: prisms
A prism is a piece of glass that turns the image from the upside down position it has when travelling through the front lens into the right one, so that we see it the way we are used to. This makes prisms an absolute and important necessity.
There are porro prisms and roof prisms. The difference is easy to tell. Porro prisms are classical: the light travels through several prisms located at different levels inside the binoculars and into the eye. Such construction makes the binoculars heavier and bulkier, also cheaper and of very high quality.
The roof prisms make the binoculars look and feel light. So, if you see two connected straight tubes, rest assured that the inside this binocular is a roof prism. The weight and the looks are, probably, the only advantages of this prism over the roof prism, the disadvantages being a much higher price and a less sharp image.
The glass used in prisms determines their quality. There are 2 types: Bak4 (very high quality) and BK7 (not as good).
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Category: lens coating
When light travels through the lens, much of is lost on the way. So, in order to keep the image bright, special chemicals are used. The lenses may have only one layer of coating, which is the least effective way. The better way is multi-coating, when the lenses are covered with several layers of the special chemicals, which makes the image much brighter. The best way is full multi-coating, when every single lens in the binoculars is multi-coated.
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Category: focus
When you look through binoculars, the image is not always sharp right away – the focus has to be adjusted. There are three common type of focusing. Center focus binoculars have a weal in the middle which when rotated adjusts the focus for both eyes simultaneously. This is the most common construction. Individual eyepiece focus demands adjustment for each eyepiece separately, but once focused it doesn’t have to be adjusted ever again, since it is adapted to your eyesight. This is very good for long distances, but not so good for short distances. Sometimes, the focus is fixed by the manufacturer and you cannot adjust it, which is not comfortable and is found in budget models.
The center focus binoculars and individual eyepiece binoculars lets you most commonly also adjust the diopter, which is very handy since human eyes tend to have different strength.
Also, with most binoculars you can adjust the interpupillary distance, or the distance between your pupils, by sliding the binoculars together or apart.
Each model has a minimum focus, or close focus, which shows the distance at which the binoculars start focusing. If the object is any closer, focusing becomes impossible.
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