What does 8×21 mean for binoculars?
Binoculars are often specified by a set of numbers such as 8×21, 7×35 or 10×40 and other. What does 8×21 mean for binoculars? The first number indicates the strength of magnification of binocular, or in other words, how many times closer the object is to you, 8 times closer, 7 times closer, 10 times closer or 12 times closer.
The second number indicates the size of the objective lens. The objective lens is the large lenses at the far end of the binoculars (i.e. not the ones you look into. This is measured in millimetres going across its diameter.
The larger the objectives, the more light is gathered by the binoculars. For example, if you double the lens diameter, this quadruples the light gathered.
1. Magnification Numbers on Binoculars Mean?
The magnification number on binoculars is expressed as part of a combination of two figures, for example 8×40 or 10×25.
The first figure (8x, 10x) refers to the power of magnification. Binoculars with an 8x magnification will make objects appear 8 times closer than they are. The higher the number, the closer objects will appear through the lenses.
Extreme magnifications (12x and up) make it difficult to maintain a steady image unless you have the binoculars stabilized on a solid object like a table or a wall. You also get zoom binoculars, which offer an adjustable magnification range.
2. Objective Lens Size (Aperture)
The number following the magnification power is called the objective lens size, also referred to as aperture. This is a critical number, because it indicates how much light the lenses are able to gather. A 10×50 binocular will have a 50mm objective lens size, a 7×35 will have a 35mm objective lens size, etc. Wider lenses usually also increase the size of the binoculars.
The wider the objective lens size, the brighter the image you’ll see thanks to more light being gathered through the lens. This number is most important for night sky viewing, or other low light conditions.
Here’s a simple question to demonstrate this: which of a 10×25 or a 10×50 binocular will be best for viewing the night sky? The answer is, of course, the 10×50. You’ll see many more distant objects in the night sky thanks to the wider aperture.
3. Understanding Field of View
Field of view (FoV) is not expressed in degrees, but in either:
- feet per 1,000 yards or
- meters per 1,000 meters
Again, a higher number simply means you’ll have a wider horizontal view through the lenses. Typically you can consider a FoV of about 300 to 375 feet as adequate. Remember though that the higher your magnification, the smaller your field of view will be due to the object being brought closer.
Finally, here’s how you convert angle of view to field of view:
Multiply the angle of view by 52.5
So, a 7.2 degree angle of view equals a 378 feet field of view
7.2 x 52.5 = 378
7.2 x 52.5 = 378
4. Understanding Angle of View
Angle of view walks hand in hand with field of view as they refer to the same thing: the amount of horizontal scenery that is visible when looking through the binoculars.
We will discuss them separately to avoid confusion, and show you how to convert one to the other if needed.
AoV is always expressed in degrees. The higher the number, the wider the area you’ll be able to see. Anything over 6 degrees can be considered a good angle of view.
If you see a really high degree number, like 72 degrees, the manufacturer may be using actual angle of view. This number is reached simply by multiplying the angle of view with the magnification value of the binoculars.
For example: a 10×50 binocular with a 7.2 degree angle of view will have a 72 degree actual angle of view (7.2 x 10 magnification).
5.Exit Pupil Number
The easiest way to understand this number is to hold a pair of binoculars about 8 inches from your eyes. The two dots you’ll see in the center of the eyepieces are where the light from the lenses hits your eyes to allow you to see the image you’re focusing on.
These need to be greater than your pupils. The value is easily calculated by dividing the objective lens diameter with the magnification. A 10×50 binocular will have a 5mm exit pupil, while an 8×25 will only have a 3.1mm exit pupil.
Anything larger than a 4mm exit pupil should be fine for most conditions, but bigger is better.
6. Eye Relief Number
The eye relief number on binoculars is mostly important only if you wear glasses. It tells you how far away from the eyepiece your eyes can be while still being able to enjoy the full field of view that the binoculars offer.