How To Measure Binoculars Power
Terms Used to Describe Binoculars
Professionals use several terms to describe the capabilities and qualities binoculars.You’ll do well to acquaint yourself with the most common of the terms before you go out to purchase your pair of binoculars for kids.
The terms include such descriptions as field of view, compact lens, low or high magnification, the exit pupil and eye relief. Also - simple and auto focus, contrast, resolution and lens coating - among others.
Several of the descriptions sound like real jargon to the ear but that should not worry you. They are easy to grasp. They all revolve around one thing - light - and how it affects the lens of the binoculars. The interaction between the lens and the natural or artificial light that occurs whenever the binoculars are in use affects the way the human eye perceives the image captured in the lens.
Please find below brief explanations on the meanings of the most important of the terms:
1. Field of View
Field of view refers to the width of the image that you see in your eyes when you look into the lens. It is the size of the area that the lens can capture.The wider the field of view, the wider the image that the kid will see by looking through the lens.
However, the image grows dimmer as the field of view grows wider because the exit pupil increases. The exit pupil measures the amount of light that enters and leaves the objective lens. A large field of view is good when capturing moving objects or when the binoculars are in motion. A very wide field of view requires lenses with a large diameter. However, these are often heavy and expensive to buy and maintain.
Magnification refers to the degree to which the object being viewed is enlarged. For example if the magnification ratio is denoted as 7X the respective binoculars will enlarge objects seven times the size that would be visible by the naked eye at the same distance.
A high magnification is not necessarily a good thing to have in a pair of binoculars.High magnification ratios dim the image. Thus, the lower the magnification ratio, the brighter the image captured, all things being equal. Also a high magnification ratio creates a smaller field of view and thus limits the width of the area (read object) that the lens may capture.
As the magnification ratio goes up, it reduces the field of view. A magnification of between 7-8X is considered as optimal for binoculars used by kids
3. Lens Diameter
It is the length (diameter) across the circumference of a lens measured in millimeters. The larger the diameter the better because this means that the lens can absorb more light and thus produce images of higher clarity.
Doubling the size of the objective lens improves the light gathering ability of a lens four times. However, when choosing the lens diameter, we must consider it together with Exit Pupil, and the use to which we intend to put the binoculars. Thus it is not always true that binoculars with large diameters are better than those of smaller size.
4. Exit Pupil
The Exit Pupil refers to the diameter in millimeters of the beam of light that leaves the lenses. A lens with a large diameter captures bright images easily. Thus a large exit pupil is more advantageous under low light conditions (including nighttime) because it enhances the capacity of the lens to absorb light in otherwise dim conditions.
This refers to the capability of the binoculars to catch details in the images it focuses on. When the resolution of a pair of binoculars is high, it will capture colors more vividly. Similarly a large objective lens is able to catch details better than a small one.
6. Eye Relief
The distance in millimeters that one can hold binoculars from the eye and still see full field of view. The longer the possible distance, the higher the eye relief. Binoculars with high eye relief are considered to be of higher quality, all things remaining equal.
The extent to which dull and bright objects can be differentiated from each other and from the target image. High contrast in binoculars helps pick out faint objects.
8. Lens Coating
Binoculars lens are coated on the inside to prevent loss of light and contain glare from the environment. The coating is intended to help manage the way light is transmitted. Coating enables the capture of contrast in the images and thus brings out their sharpness.
The deeper the coating the better because loss of light is minimized and glare is kept at bay. The coatings are denominated into three types namely;
• Coated- the lowest quality of coating and therefore least effective;
• Fully coated – the 2nd grade of coating. It is fairly effective;
• Multi -coated – the highest quality of coating. It is regarded as the most effective.
All other qualities being equal, the cost of binoculars is higher the higher the degree of coating applied on its optical lenses.
9. Inter-pupillary Distance
The inter-pupillary distance or ID refers to the distance between the centers of the eye. The distance is measured over a range of millimeters for example 58mm-75mm. Your ID must fall within the range defined for the binoculars otherwise you won’t be able to see through it well. The ID is represented by the range of distance that the barrels of the binoculars may be adjusted by being pulled in or out. It is very important to get the ID right when buying binoculars for Kids. The most popular minimum ID measure for binoculars used by children is 56mm.
10. Objective Lens
The Objective Lens is the larger of the two outer lenses of the binoculars. It is the one that collects the light that the binoculars use to capture objects. The bigger the lens the better because the more light it can gather and therefore the higher the level of clarity with which the images are captured. However binoculars with large objectivelenses are bound to be heavy and thus unwieldy for small kids to carry around.