How To Choose Binoculars For Trainspotting (Best UK Guide)

Trainspotters fall into different types, there are those that prefer to photograph every train they spot, to serve as physical evidence of their having spotted that particular train. Then there are those that prefer to record the numbers of the trains they’ve spotted in a notebook. If you’re going to record all of the train numbers you spot there’s a handy selection of books that help identify all of the types of trains along with coaching stock formations.

Platform 5 books are updated every year and contain lists of all the train classes in use on the Great British rail network. To find out more about railway publications for information on trains from the UK or worldwide check out rail-books ^[1]. To identify all of the various types of trains and other train related equipment we would suggest investing in a decent pair of binoculars.

We have been getting more and more trainspotting enthusiasts asking for advice on the best binoculars to use while trainspotting. Which is why we’ve compiled this brief guide to help you choose the right optical equipment to get the most from your hobby. You might be tempted to buy any cheap pair of binoculars from an online marketplace, but please read this article first, because when it comes to binoculars, you really do get what you pay for.

What Size Binoculars Do You Need & How Can You Tell The Size?

The first thing most people think of when they think about binoculars is magnification. It makes sense, the whole point of binoculars is to make things look bigger. But too much magnification is worse than too little in many cases. 

First up, if you choose too high a level of magnification, you’ll probably be getting a large pair of binoculars. Remember, the bigger they are, the heavier they’ll be too and it’s you that’s going to have to carry them around with you. As you know trainspotting involves lots of travel along the length and breadth of the UK’s railways which means taking everything you need with you.

We would recommend binoculars with 8x magnification are best for trainspotting as 10x will be overkill for checking out which class or engine type the trains number shows. To find out the magnification of the binoculars, look on the body of the binoculars for a series of numbers separated by the letter X, it’ll look something like 7×35, or 10×42. The first number followed by the X indicates the level of magnification and the X stands for times.

So 7x means 7 times magnification which means everything you see through the lens will be 7 times larger than seen with the naked eye. The numbers after the X are the size of the objective lens diameter. So 7×35 has a magnification of 7 times with an objective lens diameter of 35mm. In many ways the objective lens diameter is far more important than the magnification level.

Why Is The Objective Lens Diameter So Important?

There are a number of reasons why the size of the objective lens is important. The objective lens tells us;

1. How Bright The Image Will Be
   The only way for light to enter the binoculars is through the objective lens. So the larger the diameter of the objective lens, the more light can enter the binoculars and the brighter the image will be.
2. The Size Of The Binoculars
   Binoculars have 3 size classes which are determined by the size of the objective lens diameter, which are;
   Compact – An objective lens size below 30mm
   Mid Size – An objective lens between 30 to 40mm
   Full Size – An objective lens greater than 40mm
3. The Exit Pupil
   The exit pupil is the size of the diameter of light that enters the binoculars through the objective lens. It’s important to know the exit pupil size of any binoculars because it should match our own pupil size as closely as possible (especially in dull, low light conditions). 

How Does The Objective Lens Size Tell Us The Exit Pupil?

The objective lens diameter, along with the magnification,  can be used to calculate the exit pupil. To calculate the exit pupil simply divide the diameter of the objective lens by the level of magnification. Our earlier example of 7×35 has an exit pupil of 5mm, because 35÷7=5. Our pupils are around 7mm at the age of 30 and they decrease by one mm every 10 years so by the age of 40 they’ll be 6mm and so on.

Eye Relief

Eye relief is the perfect distance between your eye and the ocular lens (the one closest to the eye) and still see the full field of view. The average eye relief is between 11 and 16mm, if you need to wear glasses you’ll probably need what’s known as long eye relief. Long eye relief allows enough space to fit your glasses while still maintaining the perfect distance to see the full field of view. Long eye relief is between 16 to 24mm.

Eye cups

These are the soft plastic or rubberised edges attached to the eyepiece to create a soft edge for your face. They are usually adjustable by either twisting or folding up or down to allow slightly more or less eye relief.

The Field Of View (FoV)

This is one of those often quoted, yet often misunderstood terms that gets bandied about in binocular circles. Simply put, the FoV is all that you can see through the lens of the binoculars from right to left while looking straight forward. It’s expressed in the binocular specs as either linear or angular, and anywhere between 6° to 8° angular or 105m per 1,000m to 140m per 1,000m is considered to be good.

Types Of Binoculars

There are two main types of binoculars which are Porro prism and roof prism. They both have good and bad points but for ease of use and convenience we would recommend for getting on and off trains, roof prism binoculars to be the best choice. Below is a brief rundown of the pros and cons of each type for comparison purposes.

Pros Of Porro Prism Binoculars	Cons Of Porro Prism Binoculars
Cost less than roof prisms of comparable quality	Easy to damage the alignment of the prisms leading to eye strain & headaches
Higher rate of light transmission	Heavier
Realistic 3D image	Bulkier
Wide FoV	Hard to completely waterproof

Pros Of Roof Prism Binoculars	Cons Of Roof Prism Binoculars
More robust than Porros, due to prism arrangement less likely to get damaged	Higher production costs due to precision components. That cost is passed on the purchaser
Lightweight design	Narrower FoV
Easier to fully waterproof	Fractionally less image clarity except in top of the range models
Compact, easy to carry & conceal
Better magnification

Waterproofing Binoculars – Is It Necessary?

In a word, yes. We live in the UK where the seasons are distinguished by the temperature of the rain, so at some point, you will get caught in a downpour. If water gets into binoculars they will be rendered as less than useless. The industry has a coding system for waterproofing that was set up to eradicate any confusion into false or misleading claims.

However, this coding system is somewhat confusing itself, suffice it to say that any binoculars with an IPX6 code will be capable of withstanding any weather Great Britain can throw at them (IPX7 allows for 30 minutes submersion in one metre of water without any water entering).

Fog Proofing

There’s nothing more annoying than trying to identify some rolling stock through the binoculars only to find they’re fogged up. Leaving a warm train cabin to stand on a freezing cold platform is the perfect situation for binocular lenses to fog up. To combat this, manufacturers remove the air from the lens tubes and replace it with either nitrogen or argon gas. As these gasses contain no moisture, they cannot react to sudden changes in temperature.

Dust Proofing

Once the gas is sealed into the lens tubes it cannot escape, which also means nothing can enter either. So no dust, mould spores or microbial debris can inhibit your clear view through the binoculars lenses. Very handy in some of the train stations we’ve visited.

Light Transmission

As we said earlier on, binoculars can only work if they receive enough light to give the image brightness. A major contribution to the transmission of light in binoculars is the actual glass used in the prisms and the lenses. Some train stations can be dull on even the brightest of days so good light transmission is of great importance for trainspotting binoculars.

BAK4 Glass

This is the highest precision glass with the least imperfections (which means better light transmission) used in top quality binoculars by the best binocular brands.

BK7 Glass

This is still precision optical glass of a high quality, but with slightly more imperfections than BAK4 glass. BK7 prisms are commonly used in the production of binoculars.

E D Glass

Used for making binocular lenses, E D or Extra low Dispersion glass is designed to produce a clearer, sharper image with no colour fringing or haloing.

Lens Coatings

As well as the glass used for making lenses and prisms, there are special coatings applied to the lenses to cut down on glare, improve contrast, brightness and light transmission. Just like waterproofing codes, lens coating terms can be confusing, look for fully multi-coated lenses (FMC). FMC lenses have been coated inside and out with multiple layers. Which will not fade or wear out.

Recommended Binocular Size For Trainspotting

As we said earlier, you won’t need more than 8x magnification, but we would suggest going for as large an objective lens diameter as you’re comfortable with, certainly nothing below 25mm. An 8 x 25 pair will give you an exit pupil of 3.1mm which should be sufficient for most daytime trainspotting, obviously the larger the objective lens diameter the better, but you need to consider weight and overall size as well.

Frequently Asked Questions