Binoculars - a helpful tip

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When choosing a binocular, aside from the obvious issues such as price, size, weight, water-proof or not, there is a jargon term you may find helpful to know about:

It is called 'exit pupil' and this is the diameter (width) of the circle of light that comes out of the rear lens of the binocular - the one you put your eyes close to. This should not be confused with 'field of view' which describes the relative size the image appears at a given distance from the observation point e.g. 380/1000. This is NOT the same as exit pupil and is in fact, less important. If you look at different binocular specifications, you will sometimes see exit pupil mentioned. It should always be stated. Why is it important? You first need to understand the human eye. The front of the human eye has a lens that allows light into the back of the eye. This lens has a diameter of approximately 7mm with very little variation between individuals. Consequently, a binocular that has an exit pupil of 7mm is going to allow your eyes to get the maximum amount of light into them without them having to compensate. How is exit pupil calculated? It's pretty easy - all you do is divide the objective lens (the front lens of the binocular) diameter by the magnification factor. An example - for a binocular that is called 8 x 42 (a very common size) the exit pupil would be 42/8 = 5.25 which means that your eye is working harder to see the image from this binocular than say, from a 7x50 binocular which has an exit pupil of 7.14mm. Ever wonder why most 'marine' binoculars are 7x50's? You guessed it - because of the exit pupil. When you are standing on a boat and the motion of the ocean has you rocking this way and that, it is difficult to keep the binocular steady. The smaller the exit pupil, the harder it becomes. Exit pupil is one of those cases where bigger IS better.

So the message is - consider how and where you are going to use your binocular and if you are likely to be in an environment that is not perfectly stable (such as mounted on a tripod), a high-magnification binocular will very likely give disappointing results. Why? small exit pupil. A general rule of thumb is, the higher the magnification, the smaller the exit pupil. Consider a high-power binocular with 20x magnification and 50mm objective lens. Exit pupil would be circa 2.5mm. Quite a strain to the eye.

Binoculars with larger diameter objective lenses (42mm and higher) let a lot more light in at the expense of making the binocular heavier which means using them for any length of time quickly becomes tiring unless you have them mounted on a tripod or similar. Lightweight compact binoculars have the smallest exit pupils making them extremely difficult to use in most situations. They are next to impossible to hold steady!

Another specification that is often overlooked (no pun intended..) is eye relief. Eye relief is the depth of the shroud around the rear lenses. Some binoculars have none, other have as much as 17mm. What difference does this make? If you wear corrective lenses (glasses / spectacles) a long eye relief binocular will in most cases allow you to use the binocular without needing to remove your corrective lenses.




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