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Binouclars FAQs

What if I wear glasses?

You will probably have to pay more for binoculars if you want the same-quality image as people who don’t wear glasses.
Some binoculars have what is called long 'eye relief', meaning that your eye should be further away from the binoculars to receive maximum image quality.
If you can see a black rim around the outside of the image you observe through binoculars, that means the eye relief is too short. Either you are holding the binoculars at the wrong distance, or your glasses prevent you getting close enough to sit the binoculars in the right position.
If you wear glasses, look for either:
eye relief of 14mm or longer (17mm if you have thick glasses).
binoculars with rubber eyepieces that can be rolled back, allowing the eye to sit closer to the lens. Rather than assuming rolling back an eyepiece will work, though, it’s best to try a number of different pairs in a store before buying.
 
Focus
Do different focusing methods make a difference?
There are two different types of focusing for binoculars – centre focus and individual focus. 
. Individual focus: This the more complicated method where each eyepiece must be focused individually each time you change focus. Using this system, you simply shut the right eye and focus the left eyepiece to your left (open) eye. Then repeat the process for the right eye. If you have trouble shutting a single eye, then cover the front of the tube with your hand instead (but don’t touch the lens). When you need to focus on something else at a different distance, repeat the process again.
. Centre focus: This is the second way of focusing, where one focus control controls both eyes. However, while this sounds simpler, you still need to make adjustments for individual eyes the first time you use your new binoculars, because few people have the same vision in both eyes.
 
How to adjust centre-focus binoculars
. With centre-focus binoculars, always start by focusing the left side of the binoculars. 
. Cover your right eye (or the right lens) and adjust the centre-focus control until the left eye is in focus. 
. Now close your left eye and adjust the right eyepiece – not the centre-focus control – to your right eye.
. That leaves the left and right eyes correctly focused – the left eyepiece to the centre focus, and the right eyepiece with the correct offset for your other eye.
. And from now on, to change the focus you need only adjust the centre-focus control. The right eyepiece will remain the correct focal distance away from the left.
 
Then what are ‘auto-focus’ binoculars?
Don’t be confused, ‘auto-focus’ binoculars, also known as ‘fixed-focus’, are not auto-focus in the same manner as an auto-focus camera. What they do instead is let you adjust the magnification of your binoculars, so you can ‘zoom’ from a low magnification to a high.
This sounds useful but in practice this type of binocular works better at lower magnifications. The higher you ‘zoom’, the less light the binoculars let through. At high magnifications the image can appear quite dim.
 
What’s the best way to clean and care for binoculars?
. Moisture can be blown off a lens but don’t use your clothing, rough surfaces or Kleenex to clean a lens. They scratch easily.
. Camera and binocular stores sell lens cleaner, but try not to use it more than once a month.
. A soft camel hair brush is useful for removing grit or dirt.
. Keep the lens cap on unless you are using the binoculars, especially in sandy or windy areas.
. A soft hair brush is useful for removing grit and dirt
 
What are 'night glasses'?
These are a special type of binoculars designed for use in twilight and for looking at lit objects at night. They are not 'infra-red' or any other type of night-vision you may have seen in the movies. They simply have a good combination of magnification and objective lens size (7x50) for low-light viewing.
 
How night glasses work
One figure that is sometimes used to evaluate binoculars is the 'exit pupil,' the size of the opening in the binoculars transmitting light to your eyes.
A binoculars’ exit pupil is calculated by dividing the objective lens size by the magnification. In the case of the night glasses already mentioned, that is 50/7, which is an exit pupil of roughly 7.1. 
A young person’s pupil can expand to roughly 7.1mm, possibly as wide as 8mm. So night glasses provide very close to the maximum light that person’s eye can use. This means that even in dim light, night glasses can seem surprisingly bright.
However, some of that light is not necessary for people who are no longer in their twenties. A middle-aged person’s pupil will be closer to 5mm, and the elderly 4mm. In those cases, binoculars with a lower exit pupil will not restrict night viewing.
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