Binoculars Specifications Explained

Binoculars are an essential piece of kit carried by almost all researchers and wildlife watchers. When selecting a pair, the range of models available on the market can be overwhelming. Long lists of unexplained specifications and acronyms can further complicate the decision.

In this article, we aim to clearly explain what each of the key technical specifications means, why they are important and how they apply to different uses.

Magnification

All binoculars will be labelled with a combination of two numbers such as 8x32 or 10x42. The first number denotes the magnification of the binoculars, also known as the ‘power’. The magnification essentially tells you how many times closer the image will appear when viewed through the binoculars.

8-10x magnifications are generally the most popular for watching wildlife, although this will depend on the specific species and habitat you are monitoring.

It is important to understand that the magnification has a knock-on effect on many other specifications:

• The higher the magnification, the smaller the field of view (see FOV below).
• A smaller FOV means you will be more susceptible to a shaky, less stable image with higher magnifications.
• Higher-magnification binoculars also tend to be slightly heavier

For general wildlife watching, monitoring or research 8x magnification binoculars are probably the way to go. They offer a perfect balance between magnification and field of view that is ideal for following fast-moving animals such as birds and mammals.

For situations where you require a closer look with more detail visible e.g. for identification purposes, a 10x magnification can be worth considering. They are best used for slower-moving animals due to their smaller FOV, which can make rapid scanning more challenging.

Objective diameter

The second number listed in the configuration (e.g. 8x42) is the objective lens diameter in mm. The objective lenses are those at the far end of the binocular, as opposed to those held up to your eyes.

The general rule is that the larger the objective lenses, the more light they will collect and therefore the brighter the image. The downside is that larger lenses usually lead to a heavier pair of binoculars. When choosing binoculars you must find a trade-off between these two and take into consideration how you want to use them.

The most common objective diameter for wildlife-watching purposes is 42mm. You will also commonly come across people using 32mm options who want a slightly more lightweight option. Higher diameters (e.g. 50mm +) tend to be a little too heavy for most wildlife-watching purposes unless using a tripod.

Binoculars that offer an objective diameter of less than 30mm (common dimensions are 25mm and 28mm) are known as ’compact binoculars’ and are more suited to travelling or for quick spotting e.g. alongside a scope.

At the time of writing, NatureSpy offers the most popular configuration - 8 x 42. If you are looking for an alternative size, please get in touch.

Field of View (FOV)

The Field of View (FOV), also referred to as the linear field of view, tells you the size of the observable area when looking at a certain distance. The size of this area is usually measured in either feet or metres at a set distance of either 1000ft or 1000m.

You may see the viewable area described as an angle by some manufacturers e.g. 8° - this is known as the angular field of view or AOV.

A smaller field of view limits the area you can see, so can make wildlife spotting tricky.

Models with wide fields of view will be particularly helpful when following fast-moving animals, or when aiming to monitor a large area. Hand shake will be less noticeable, meaning they are also a better choice when positioned on a boat or other moving vehicle.

As mentioned above, magnification and field of view go hand-in-hand. For an 8x42 pair of binoculars, a field of view of around 400ft or more can be considered a wide field of view. Both the Bushnell Engage EDX and Forge 8x42 offer a wide 426ft FOV.

Coatings

The majority of the acronyms listed in a binocular specification (FMC, MC, PC-3 etc.) will be referencing the many chemical coatings applied to the binocular lenses and internal components (prisms). You may find different manufacturers using varying terminology, causing further confusion!

1. Optical coatings

Optical coatings aim to improve overall light transmission and clarity, optimising the quality of the resultant image. Higher-end binoculars tend to be treated more thoroughly, providing understandably sharper and brighter images.

Some of the common terms you may find listed are as follows:

• Fully coated (FC) - all lenses have been coated (on the air-facing side)
• Multi-Coated (MC) - at least one surface has been coated multiple times
• Fully Multi-Coated (FMC) - all surfaces have had multiple layers applied.

DiElectric - a coating on the internal binocular prisms which assures high internal reflection for vivid, realistic colour.

Phase coating (PC-3) - a coating on internal prisms to reduce chromatic aberration*.

*Chromatic aberration occurs when different colours of light passing through the prisms or lenses fail to focus at the same point. It can make images appear fuzzy, especially around the edges of items where red or purple outlines can be seen.

Side by side comparison of two digiscope photos, taken through the Bushnell Forge (left) and the Bushnell Prime (right). The inset zoom square on each shows the same area of trees. The left binocular is a higher-end model with more coatings so shows a sharper image with more true-to-life colouration. The right, more affordable binocular has some red colour fringing around the tree edges. Note also the difference in field of view.

2. Protective coatings

In addition to the optical coatings, most binoculars will be treated with a protective coating to prevent external damage to the lenses. Such treatments can provide a strong layer to reduce scratching, while also repelling dust, oil and debris.

Some manufacturers use a coating that encourages the beading of water (e.g. Bushnell’s EXO barrier) - a very helpful feature allowing the lenses to be cleared quickly in rainy conditions.

Eye Relief

The ‘eye relief’ is the ideal distance, measured in mm, between your eyes and the lens. At this distance, you will be able to see the full image. Almost all binoculars offer the ability to adjust the distance between the lens and your eye by moving the eye-cups. Most modern binoculars use a twist design however, you may still find pairs with foldable eye-cups on the market.

Glasses wearers can have difficulty when their eye-glasses stop them from getting their eyes close enough to the binocular lenses - and can therefore see an incomplete image, with kidney-bean shaped areas of blackout. This can also be an issue for non-eye glass wearers if they do not have the eye-cups in the right position.

Most glasses wearers will need binoculars with an eye relief of at least 15-16mm. This will allow them to keep their eyes a small distance from the lens (caused by the glasses) while still seeing everything. Eyeglass wearers should usually keep the eye-cups twisted all the way down.

Non-glasses wearers will likely need to keep the eye-cups either partially or fully twisted out, in order to get the full unobstructed picture.

Close Focus

The minimum distance at which an observed subject will be in full focus is described as the ‘close focus’. Anything closer to the binocular than this distance will appear blurred.

For average wildlife watching, the close focus may not be the most important consideration. However, for observing nearby birds, insects and plants a good close focus can be a vital feature. For such purposes, a close focus of under 2m would be best (such as the Engage EDX, which offers a 1.8m close focus).

Hopefully the advice presented in this article will help you understand the specifications of binoculars. However, if you have any further questions please just get in touch.