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.

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.

To keep things simple, binoculars are classified in 3 sizes. These are small, midsize, and full-size.  These are categorised as:

If you have this set of numbers on your binoculars it means they have a magnification of 12 times and a lens diameter of 50 millimeters. The larger the lens diameter is the more light can enter the binoculars which will allow you to see objects clearer. 12 times magnification is also the largest magnification it is possible to have on binoculars without having a stand to steady them on, due to shaky vision.

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.

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.

8×30 on binoculars means they have a magnification of 8 times and a lens diameter of 30 millimeters. Which means objects will appear 8 times closer through the lens and the lens diameter helps to calculate the amount of light that can enter the binoculars.

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.

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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.

The 20x is the amount of magnification (20 times) and the 50 is the objective lens diameter (how much light enters the lens).

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Objective lens diameterbinoculars

The 10x indicates 10 times magnification and the 50 indicates the lens diameter of 50 millimeters. The larger the lens diameter is the more light can enter the binoculars which allows you to view objects in lower lighting conditions.

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.

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.

Objective lenssizes microscope

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.

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The numbers indicate the magnification size and the objective lens size. The first number is the magnification (in this case 8 times) and the second number is the objective lens diameter (in this case 42).

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.

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.

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.