Binoculars
When choosing binoculars, it can be very confusing when people start referring to the 'exit pupil', 'field of view',
'objective lens', and so on.
The key to choosing binoculars is understanding binocular terms, and how each should affect your decision in
selecting the binoculars which are right for you.
Binocular Terminology
For most people the price will be the prime consideration when choosing binoculars but first of all, let's
explain the numbers and terminology. Whatever your budget, you still need to know what to look for when choosing
binoculars, monoculars or spotting scopes.
Exit Pupil Size
The Exit Pupil is the size, in millimetres, of the beam of light as it leaves the eyepiece of the binoculars.
It is very easy to work out. Just divide the size of the objective lens by the magnification.
So, a 10 x 50 binocular will have an exit pupil of 5mm.
If you are choosing binoculars for general-purpose use, an exit pupil of between 3 mm and 5 mm is good. More
than 5 mm would be good if they were going to be used mainly at very low light levels.
Field Of View
What about the Field of View, sometimes referred to as FOV? This is usually measured as an angle in degrees, but
can also be given in feet. It tells you how much of a scene you will be able to see.
If you are choosing binoculars for close-quarter use (such as in heavy woods), or when the subject or the user
is moving, a larger FOV is desirable.
Depth of Field
The Depth of Field can be an important consideration when choosing binoculars. This refers to how much of a
scene will be in focus - i.e. the distance from the nearest object in focus to the furthest object in focus.
The smaller the depth of field, the harder it will be to focus. The higher the magnification, the shallower the
depth of field will be. Also, the depth of field gets smaller, as the distance to the subject gets shorter.
Infinity Focus
Focus-free binoculars (sometimes known as infinity-focussed binoculars) are usually focussed on infinity with a
very large depth of field (and usually a wide field of view).
This does mean that you will be unable to focus on objects closer than about 20 to 30 feet, but are ideal for
activities where the subject is moving, like motor sports.
Eye Relief
Eye Relief can be an important consideration when choosing binoculars, especially for spectacle wearers, or if
you want to be able to use them while wearing sunglasses. This can affect the field of view, so you will have to
decide which is more important.
Lens Coating
Lens coating is very important. There is a lot of confusion about the different colours of lens coatings, but
basically, all of them reduce reflection and so increase the amount of light captured by the binoculars. This, in
turn, increases the brightness and clarity of the image.
For a lot of people one of the most important considerations when choosing binoculars is price. What is the
difference between a pair of binoculars costing, say, 50, and a pair with the same designation costing £100
or even £1000?
Generally speaking, you get what you pay for, and the more expensive binoculars will be made from higher
quality materials. They will give higher quality images - sharper and brighter. They will probably be a bit more
rugged.
But, you don't have to spend a lot to get a good quality pair of binoculars.
You can spend a lot more money, but only get a slightly better image, and unless you have extremely good
eyesight, you may find that you can't actually tell the difference.