The numbers on binoculars tell you their strength (magnification) and their size (objective lens diameter). For instance 10×42 means they have a magnification of 10 times and an objective lens size of 42mm.
The objective lens size gives you an idea of the size of the binoculars and how much light they can gather. Once you understand the meaning of these 2 numbers and how they apply to your viewing, you will be able to choose the right size binoculars for your purpose.
What Are The Different Binoculars Sizes?
To keep things simple, binoculars are classified in 3 sizes. These are small, midsize, and full-size. These are categorised as:
- Compact – Less Than 30mm Lens Size (8×25, 10×28 etc)
- Midsize – 30 To 40mm Lens Size (10×30, 8×32 etc)
- Full-Size – Larger Than 40 mm Lens Size (8×42, 10×50 etc)
The lens sizes that are used in these classifications are the second numbers in the model names. Due to differing designs, optics and build, models with the same size objective lens size can vary in weight and size.
Compact models are convenient for storing in backpacks as they are lightweight and easily transportable. Midsize binoculars are more suitable for long term usage, as they are a comfortable size and weight for most people and they will have a clearer, brighter image. Full-size binoculars are often too heavy for long term usage unless supported with a tripod.
What Is The Magnification Number, And What Does It Mean?
The magnification number is the first number on the binoculars, the one followed by the x, which stands for times. For instance, 10x means a magnification of 10 times. So any item looked at through a 10x binocular will look 10 times closer than it is. Most binoculars will have a magnification of between 8 and 12 times, any higher than this would result in poor vision due to the effect of shaking the binoculars which is inevitable.
What Is The Field Of Vision?
With binoculars, the higher the magnification, the lower the field of vision. The field of vision allows you to locate the object you are looking for, so if your field of vision is low, it will be harder to spot your object. It also means there is less light allowed into the lens, which in turn means a darker image. This is reflected in the second number on the binoculars.
So the field of vision is the area you can see through the lens of the binoculars and is how you locate whatever it is you are trying to see.
What Does Objective Lens Diameter Mean?
The diameter of the objective lens tells you how much light is entering through the lens. The higher the second number is, the more light the lens can gather. The amount of light the lens gathers affects the brightness of the image that you can see through the lens. Always go for the highest objective lens diameter you can, because this will give you a clearer image.
What Does Eye Relief Mean?
The eye relief is basically the distance between the eyepiece and your eye whilst the complete field of view is visible. This is especially important if you wear glasses. Most modern binoculars have eyepieces that are adjustable usually by twisting to shorten the eyepiece or by rolling back the soft rubber eyecups.
What Is The Exit Pupil?
Basically, the higher the number, the brighter the image. So a higher number, means seeing clearer in lower light. Which in turn means it will be easier to still see the full image if your hands shake.
The exit pupil size is found by dividing the diameter of the objective lens by the magnification number. For example, 8×42 binoculars gives an exit pupil number of 5.25mm.
42÷8=5.25
This figure indicates the size of the shaft of light that gets to your eyes. 2mm is roughly the size of pupils in bright light and 7mm is roughly the size of pupils in complete darkness. So in low-light situations (dawn, dusk or heavy tree cover), having a high exit number can make viewing that much easier. In bright daylight, exit pupil size is relatively unimportant as almost all binoculars exceed 2mm exit pupils.
