what do the numbers mean when buying binoculars?
May 30, 2010 by
Filed under Binoculars Ratings
I am looking for a good set of binoculars for hunting. Can anyone help?
Comments
5 Responses to “what do the numbers mean when buying binoculars?”Speak Your Mind
Tell us what you're thinking...and oh, if you want a pic to show with your comment, go get a gravatar!
they mean the zoom on the binoculars on them
They categorize binoculars by the magnification × the objective diameter; e.g. 7×50.
Magnification — The ratio of the focal length of the eyepiece divided into the focal length of the objective gives the linear magnifying power of binoculars (sometimes expressed as “diameters”). A magnification of factor 7, for example, produces an image as if one were 7 times closer to the object. The amount of magnification depends upon the application the binoculars are designed for. Hand-held binoculars have lower magnifications so they will be less susceptible to shaking. A larger magnification leads to a smaller field of view.
Objective diameter – The diameter of the objective lens determines how much light can be gathered to form an image. It is usually expressed in millimeters.
the first number is the zoom, the second is the diameter of the larger lens
* The first number – the 8 in an 8×40 or 8 in an 8×25 – is the magnification. Magnification simply means how many times the image is enlarged over normal when you look through the binocular. Another way to think of magnification is how many times closer the subject appears to be in a binocular than it actually is. An 8×40mm, then, with 8x magnification, makes the subject appear to be eight times larger or, to look at it the other way, eight times closer than it really is. An 8×50, 8×40, 8×20 are all 8x binoculars and all magnify an identical 8x.
In a zoom binocular, the first two numbers out of three, represent the zoom range. In a 7-15×40, for instance, you can change the magnification from 7x up to 15x.
Choose this first number carefully – more is not always better and this is especially true of magnification. As magnification goes up, image brightness goes down, field of view (the amount of territory seen through the binocular) goes down and, just as importantly, image steadiness also goes down. In fact, image steadiness is the limiting factor on how much magnification can be used in a handheld binocular. This is because a binocular magnifies not only the image, but also every shake and tremble in your hands. In general, most people find that a 10x binocular is the highest magnification that can be comfortably steadied without a support. Anything higher than a first number of 10x requires additional support, usually in the form of a tripod.
* The second number in a binocular – the 40 in 8×40 or 25 in an 8×25 – is the diameter (width) of each front lens, measured in millimeters. This number directly affects performance. When magnification and quality is equal, the larger the second binocular number, the brighter the image as well as the sharper the image. An 8×40, then, will produce a brighter and sharper image than an 8×25, even though both enlarge the image an identical eight times. The larger front lenses in the 8×40 also produce wider beams of light (exit pupil) that leave the eyepieces. This makes it more comfortable to view with an 8×40 than an 8×25.
On the other hand, the larger front lenses in the 8×40 binocular also make the 8×40 bigger and heavier than the 8×25 binocular. An 8×25 may not be optically as good as an 8×40, but its smaller size makes it a better traveling companion. Remember, a compact binocular will outperform any full size binocular left at home or in the car because it was too heavy to carry.
By the way, the second number is not the field of view as many people mistakenly believe, nor does it affect the field of view.
Let’s say you are looking at a pair of 10×42mm binoculars. The 10x refers to the magnification. In this case, 10x. The 42mm refers to the objective diameter, in this case 42mm. If you divide the diameter by the magnification, you get the exit pupil. In daylight your pupil is 3-4mm. You want an exit pupil no less than 3mm. 4mm is much better, otherwise you will see the edges blacken. At night, your pupil opens to at least 5mm. You want an exit pupil no less than 5mm.
The larger to diameter, the more light gathered. On a clear night, look up. You see plrnty of stars. Then look up with a x50mm pair of binoculars. It is like habing 50mm pupils. You will see many more stars, too faint to be seen with the naked eye.