Understanding Binocular Numbers

Binoculars are described with numbers. That is the only way we are able to differentiate them and know what is what. But the question is; what do the numbers mean and what makes one binocular better than another and one binocular better fit for some activity than the other?. Understanding binocular numbers is important before you are able to choose the right type of binoculars.

Now let’s look at what the numbers really mean.

understanding binocular numbers
Binoculars Magnification and Field of view

What do Numbers for Binoculars mean?

1.Magnification and Objective lens Size

Every binocular has some numbers printed on them which look like these; 10×50 or 8×42. The first number is the magnification power of the binoculars. This number tells you how much the object is magnified. Let us take the first example; ’10×50′. In this example, the magnification is 10. This means that the object that you are viewing will appear 10x bigger than your unassisted eyes will see. The higher the magnification the steadier your arms need to be to obtain a clear image.

The second number from the example above represents the objective lens diameter size in millimeters. The objective lens which is also called the field lens is the bigger and further lens on the binoculars when you are viewing.

This size is the determinant factor on the light gathering power of the binoculars. The bigger this number, the heavier your binoculars will be but the better its light gathering power. Making your images appear brighter and clearer. This also increases the price and the weight as well. Such binoculars are also able to see further than their compact counterparts which have a smaller objective lens. You will often find bigger binoculars are used in astronomy than maybe hunting, bird watching, hiking, etc.

You might also see numbers like these;10-30×50. Such numbers tell you that the binocular is having a variable zoom. What it means is that it can magnify the object in view by any amount from 10 to 30 times. You can vary your magnification from the first number to the last number.

Some people do not like binoculars with variable zoom because they said that whenever they change magnification, they also have to change the zoom to get clearer images.

You might have also seen binoculars with magnification like this 25/40×100, what this means is that the binoculars used have multiple fixed power eyepieces to get different levels of magnification.

  • field of view

The field of view is either expressed in degrees, meters or feet. This tells the angle visible or the number of meters visible at a range of 1000m. If it is an angle, the angle you see is just a part of the 360 degrees which will be the complete view. It is the same if it is denoted in feet. The same saying goes for 578 feet at 1000 yards.

To get the angle in degrees, just divide the number of feet, in this case, 578 by 52.5 to get a bit more than 11 degrees

  • Eye relief

The eye relief is the distance between your eyes and the binoculars eyepiece to be able to see a clear image through it and it is measured in mm. This number eyeglass wearers also need to pay attention to. Since the glasses move your eyes back a little bit than non-glass wearers. Therefore, choosing binoculars with long eye relief is better. As they have room for your glasses as well.

When using binoculars with an eye relief of less than 10mm you’re only seeing the center of the image. the bigger the eye relief, the better and comfortable viewing through them will be. Most binoculars will have an eye relief of 8-13mm.

  • Exit pupil

This is another important factor in determining how well binoculars work in low light conditions. Exit pupil size is the diameter of the actual beams of light leaving the eyepieces.

It’s calculated by dividing the diameter of the objective lens by the magnification. So 10×50 binoculars would have an exit pupil size of 50/10 = 5mm.

As we get older the size our pupils open too decreases. An observer in their 50’s may only have pupils which open to 5mm for example. To these people, large exit pupils offer little advantage.

In daylight, your eye’s pupil will contract to a size of 2-4mm. But at night they’ll open to 7mm. This varies with your age as well. If the light beam is wider than your pupil if can’t all get through, so binoculars with 4mm exit pupil size seem as bright as those with 6mm in daylight. But in poor light, your pupils may open to 6mm, so the binoculars with a 4mm exit pupil seem dimmer than those with 6mm.

So what do this numbers actually mean? What it implies is that for daytime observation, your exit pupil size isn’t much of an issue. For dusk (wildlife) or night (astronomy) use a larger 6-7mm exit pupil is a real boon.

Finally, as we can see understanding binocular numbers is important in deciding what kind of binoculars to buy and what the specifications they must have.

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