Understanding Magnification Power and Aperture in Binoculars

magnification power and aperture in binoculars

Magnification Power and Aperture in Binoculars

Binoculars bring distant birds, and other wildlife, up close, allowing the careful study of shapes, colors, and patterns. If you are shopping for a pair of binoculars, making the right choice means understanding the magnification power and image brightness, and how these two elements influence the field of view, image stability, and weight of the binoculars. Magnification power and aperture can make or break your birding or outdoor experience. On this note, I provide a basic review of magnification power and image brightness in binoculars so that you familiarize yourself with basic concepts before choosing the right binoculars for you.

Elements to Consider

Binoculars for bird watching or wildlife observation should enable you to quickly find and focus on stationary or moving targets, as well as, maintain such targets within the binocular’s field of view even as it moves. Also, the image should be bright enough to allow recognition of colors and patterns in the bright light conditions of open habitats and also in the dim light of a forest interior.

Magnification power (magnification) and the Aperture or diameter of the objective lens in millimeters (aperture).

Magnification X Aperture

These two elements are indicated as two numbers separated by an “x” or multiplication symbol such as 8×40. The magnification number and aperture are generally indicated on the binocular’s focusing wheel. Thus 8×40 binoculars magnify or bring a bird eight times closer to you, or get you 8 times closer to the bird. The 40 millimeters aperture is the diameter of the objective lens or area that captures light for the image in your binoculars.


The objective lens captures the image and provides the light, which is magnified by the ocular lens in front of the binoculars.

What is the Magnification Power, and how it works?

The magnification number tells you how many times your binoculars get the subject up close to you.

For example, if you see a bird within 47 yards from you, binoculars of a magnification power of 8x would bring that bird eight times closer; or you would see the bird as if you were standing within 5.8 yards of that bird without binoculars.

Distances that magnification powers of 10x, 8x, and 6x would bring a subject (bird) located within 47 yards of the observer. Distances from the observer= 47 yd/10x, 8x, 6x. Magnification power affects the size of the field of view and the stability of the image, both of these concepts are discussed below.

What is the Aperture?

The aperture is the diameter of the objective lens and controls the amount of light that enters the binoculars. The wider, the diameter the brighter the image. The tradeoff is that wider apertures mean wider objective lenses and consequently larger and bulkier binoculars.

Focusing Knob

The focusing knob is the wheel that allows you to focus or sharpen the image. While most binoculars on the market have a center focusing knob that controls the focus for both barrels, some models have separate focus adjustments on the two barrels. Binoculars with separate focus adjustments are too slow to be useful for bird watching.

Magnification power and the field of view?

What is the field of view? It is the size of the viewing window or area encompassed by the binoculars.

Binoculars with higher magnification get you close to the subject but reduce the field of view.

Beginner birders lean towards binoculars of high magnification power under the impression that the more magnification, the more up close a bird will be seen. While this is true, high magnification powers result in a smaller field of view, which makes it difficult to find and keep track of a moving bird in the field.

field_of_view binocularsUsing the example of a bird within 47 yards above, these sanderlings (Calidris alba) show fields of view seen through binoculars with magnification powers of 6x, 8x, and 10x. Higher magnification power results in smaller fields of view. 

This short video covers the relationship between magnification power, aperture, the field of view, and image stability.

Magnification Power and Image Stability

High magnification power (10x and 12x) get the subject closer to you but reduce the field of view, affecting image stability.

It can be said that binoculars that get the subject 10 times closer to you reduce the field of view proportionally and magnify the small movement of your hands and body about ten times, resulting in a shaky image. An unstable image and narrow field of view can make finding and tracking a moving bird a challenge, particularly in foliage or interior forest conditions. A jerky image can be frustrating and quickly leads to eye and brain fatigue unless you have your binoculars on a stabilizer such as a tripod.

magnification power and aperture in binoculars
Adjustment of eye relief distance. (A) Eyepiece was expanded to obtain the correct eye relief distance. (B) Expandable eyepieces. (C) The eyepiece was retracted to account for the distance created by the eyeglasses.

Eye Relief

Eye relief is the fixed distance between the ocular lens and the user’s eye at which the user can see the full field of view. The eye relief varies between binocular models and sizes. If a viewer’s eye is too close or too far from the ocular lens, a reduced field of view will be obtained. Eye relief distance can be particularly important for eyeglass wearers because eyeglasses create a minimum distance between the eye and the binocular eyepiece. Binocular models have adjustable eyepieces or retractable cups to create the right distance between your eye and the eyepiece.

For someone who wears eyeglasses, binoculars with an eye relief of 15 mm or more are recommended. Eye relief specifications are indicated in the specifics for each model.

Other elements to consider

Binoculars can have a glass or plastic lenses. Although most binoculars have glass lenses, which are considered to be of better quality, the lens covers on glass or plastic lenses can make both lens types produce similar image qualities.

Check the manufacturer’s reputation and guarantees. Consider how long the manufacturer has been in the business and what other products it manufactures, as well as how it will handle claims if the binoculars are damaged.

Porro Prism and Roof prism binoculars have the prisms inside aligned differently. Each type of alignment has its own pros and cons regarding image quality, durability, waterproofness, and cost. Roof prism binoculars have a modern look, and are more durable, but are more expensive. Porro prism binoculars have a traditional look where the barrels are narrow and then expand to wider barrels.

Waterproof or not. Depending on the use you will give to the binoculars, see if you need binoculars resistant to water or humidity and choose the appropriate model.

What is the ideal pair of binoculars?

If you are going to spend a lot of time carrying your binoculars in the field, you need a pair of lightweight binoculars that allow you to quickly find and focus on small and moving birds in various light conditions. Then, consider the binoculars’ field of view and aperture:

The binocular’s field of view: Magnification powers of 7x and 8x are the most recommended and used by bird enthusiasts of all levels of expertise. These magnification powers will ensure that you have a wide-enough field of view that allows you to find, focus, and keep track of moving birds.

Image brightness or aperture: Apertures or objective lenses of diameters between 32 to 42 mm are the most recommended. Diameters of 40 to 42 mm are the most popular. You want enough light in your image to see colors, patterns, and subtle features, particularly in dim light conditions.

Binoculars with high magnification power, such as 10x and 12x, get the subject up close but will have a narrow field of view, and shaky images as your body movements are also magnified.

Binoculars with wider diameters than 42 mm tend to be larger and bulkier, which may become a problem to carry around for extended periods.

An Audubon Magazine quote based on opinions of experienced birders indicates that: “Most birders prefer 7x or 8x magnification power binoculars because they’re bright and have a wide field of view, making it easier to find birds and to follow them in flight.”

The magnification power and apertures most recommended include 7x4o, 7×42, 8×40, and 8×42.

What magnification power and aperture do you use? Are you happy with what you have?

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