Before purchasing a pair of binoculars for bird watching, it’s important to know what the numbers used to describe them mean. Although, at first they may seem confusing, these numbers give a lot of different information about the type of binoculars and understanding them will help you find a pair best suited for bird watching.
All binoculars are described using a pair of numbers such as 8x 40, or 10x 50. The first number represents the magnification, also referred to as the power or zoom, and shows by how many times objects viewed through the binoculars are enlarged. For example, binoculars with a magnification of 8x will make an object appear 8 times larger or closer than it actually is when viewed with the naked eye.
When buying a first pair of binoculars, many people choose a pair with the highest magnification possible assuming that this will give them the best and most detailed view. However, big does not always mean better, especially if you’re planning on using your binoculars for bird watching.
What do the numbers mean?
Although binoculars with a high magnification will make the birds you’re looking at appear much larger, they’ll also give you a small field of view.
The field of view is the width of the area seen through your binoculars and is determined by the design of the eyepieces. It can be expressed in two ways; the width in metres at 1,000 metres, (or in feet at 1,000 yards (420 feet)), or in degrees of field.
The field of view when expressed in metres is called linear, and when expressed in degrees is called angular. To calculate the angular field, you need to divide the linear field by 17.5 for metres and by 52.5 for feet.
For bird watching, you want a wide field of view at around the 7.5º (130m/400 ft) or more mark. This means that when you look through your binoculars you will see a large enough area to be able to easily locate small and fast-moving objects like birds. If the field of view is narrower, then the area visible through your binoculars will be smaller. The bird may look bigger but by the time you’ve moved your binoculars around to find it, it may have flown away.
If you see a pair of binoculars marked with a really large field of view, say 72, this means the manufacture is using the actual field of view. This has been calculated by multiplying the angular field of view by the magnification, so if you want to find the angular field of view you need to divide by the magnification. Eg 10x 50 binoculars with a 72 field of view will have a 7.2 degree angular field of view.
Binoculars with a zoom of 8x are popular with bird watchers as they give a wide field of view and also offer good magnification. If you are more experienced with using binoculars or you are going to be looking at birds from a far distance such as waterfowl on a lake, or birds of prey high up in the sky than a magnification of 10x should be ok. Any higher, and the field of view will become significantly smaller making it more difficult to find the birds.
If you’re still not convinced to go with a lower magnification when choosing your binoculars there are a couple of other reasons that should persuade you.
Less bright, less stable
Increasing magnification can also have an impact on the brightness of the image. The second number that describes a pair of binoculars is the size of the objective lens, which is the lens closest to the object you are looking at. Light from the object enters the lens and focuses it at a short distance behind. The larger the effective diameter of the objective lens, the more light it collects which makes for a brighter image.
8x 42 binoculars have an objective lens that measures 42 mm, while 10x 50 binoculars have an objective lens of 50 mm, which is usually the maximum for handheld binoculars. Any bigger and the binoculars will be too heavy to hold conformably.
However, if the objective lens stays the same but the magnification increases the image will become less bright because of how magnification and the objective lens control the size of the exit pupil.
The exit pupil is a virtual aperture that regulates the amount of light that is delivered through the binoculars to your eye. The exit pupil is calculated by dividing the size of the objective lens by the magnification. A smaller exit pupil will allow less light in so the images will be less bright. This means that 10x 42 binoculars have an exit pupil of 4.2 mm compared to 8x 42 binoculars which have an exit pupil of 5.25 mm.
Generally, if the exit pupil is smaller than the diameter of the pupil of the human eye (about 2-4 mm in the light and 4-8 mm in the dark), the image will appear less bright. Very high-power binoculars such as 15x 56 have an exit pupil of just 3.7 mm. Not only will these produce dimmer images, but they will be too heavy to use for any length of time without a tripod.
High-powered binoculars can also be difficult to stabilise. This is because every shake of your hand or gust of wind will be magnified as much as the image. A high level of magnification will mean your images are less sharp, so although you’ll get a more close-up image of the bird, you’ll find it difficult to pick out identification details. Again, if you need a high level of magnification, then you’re probably going to need a tripod to stabilise your binoculars.
Some binoculars come with variable zoom which means you can adjust the magnification. But these are not recommended as the images tend to be poorer and they are more susceptible to developing faults.
Binoculars Magnification Quick Guide
12x and higher – long-distance viewing from a fixed point. Usually mounted on a tripod to reduce the chance of the image shaking. Good for sporting events or theatre viewing but can be used to view birds from afar.
8x – 10x – the ideal magnification for bird and wildlife watching. They have the most versatile combination of magnification, stability, and image brightness, and can be hand-held. The best magnification for general bird watching.
7.5x and lower – useful for people who find it hard to keep higher magnification binoculars steady. Wide field of view is good for observing fast moving wildlife, for example insects, at close range.
Zoom binoculars – not recommended. Images will be lower quality and they are more likely to develop faults. Avoid.
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