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.
So first of all, lets explain the numbers and terminology.
This article refers to binoculars, but the terms apply to monoculars, spotting scopes and telescopes as well.
All binoculars will have two numbers, separated by an ‘x‘, usually termed the designation. For example, 7 x 42 or 10 x 25.
The first number is the magnification.
So, a 7 x 42 pair of binoculars has a magnification of 7 – it will make the object you are viewing look to be 7 times larger, or 7 times closer.
To put it another way, an object 70 yards away will appear to be only 10 yards away.
The second number is the size (diameter) of the objective lens (in millimetres). The objective lens is the front lens of the binoculars.
Why is this important?
Because it will affect the brightness of the image.
The larger the front lens (objective lens) of the binoculars, the more light is gathered, so the image will be brighter and clearer. Remember though, that bigger objective lenses mean bigger and heavier binoculars.
So now we know that a 10 x 50 pair of binoculars will magnify the image 10 times and will have a front lens 50 mm in diameter.
Similarly, an 8 x 21 monocular will magnify the image 8 times and will have a front lens 21 mm in diameter.
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 Binoculars
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 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 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.