Magnification of a view brings the image closer, so it appears larger than when seen without binoculars. How close the object needs to be brought is more dependent on the amount of detail the viewer wishes to see. The ability to be able to identify a particular bird from a distance of 500m is different to looking at it in close observational detail.
10 times magnification makes an object at 1000m look the size it would appear if the viewer stood 100m away from it. Something too close to the field of vision will appear distorted and blurred under magnification. The magnification has to suit the subject under observation and the distance it is located from the binoculars' user. The most popular levels of magnification in binoculars are 8x and 10x, which offer a wide view field and reasonable detail. Higher than this level without a tripod mount is impractical because of the exaggeration seen in hand shake.
Extreme High Zoom Magnification
A pair of standard binoculars may have observational lenses with a 50mm diameter, which is a measurement on the border between regular binoculars and telescopic/astronomical size. They might also feature magnification up to 100x. In practice this level of magnification would narrow the field of vision and reduce the exit pupil so much (to 0.5mm) that hardly any light would reach the eye.
The exit pupil is important as it indicates how much usable light enters the eye. As the human eye responds to light and dark by increasing or reducing pupil size, anything above 7mm (the average maximum of an enlarged pupil) will produce light which the human eye ignores and does not process. The light on the periphery of the 7mm boundary is wasted. If the exit pupil presents light through an aperture smaller than the minimum the human eye closes its pupil (2mm-3mm), it is too dark to see.
When the binoculars are held at arm's length and pointed at a bright light, a small circle is observable in the centre of each eyepiece. This is the exit pupil. The wider it is the brighter the view field will appear because of the amount of usable light available to the eyes.
Quality Construction and Materials Matter
Magnification is not the only thing that makes a pair of binoculars good for seeing distant objects. Quality materials and construction add much to the viewing experience and the performance of a pair of binoculars.
Lenses and prisms may be made from glass or some other material such as polycarbonate. Optical-quality glass with a top finish resulting from skilled grinding and polishing, coated with anti-reflection treatments is the best choice for transparent parts. The barrels hold the lenses and prisms in place, and their construction is also important. Robust yet lightweight material is ideal but is expensive, so a compromise is made between the benefits. Cheaper plastics might be used in place of fine metals and thicker skins, creating more weight, are used for better protection. The lens barrels could be made as either one or two piece constructions. The sensitivity level of the focus mechanism can vary widely. Prism mounts and retention mechanisms could be school-project cheap or scientifically sound depending on the types of material used and how parts are assembled. All of these factors contribute to how waterproof a pair of binoculars might be. Any moisture ingress can cause fogging. Outside, water repellent properties on the glass are important so that observations can continue regardless of the weather conditions.
Optimising Binoculars Use
Whatever binoculars are used for, indoors or outdoors, they help to find things. Scanning an arena to spot a particular artist or athlete, a theatre to get a closer view of the stage and the actors' emotions, or woodland to capture an animal in its habitat will require knowledge of distance and appropriate focus settings. Handling the equipment with ease and comfort is learned through practise. As confidence develops, the switch from one magnification to another is readily performed with smooth transition as the image is brought closer for detailed scrutiny.
Shape to Suit
Binoculars are offered in three basic shapes. Traditional binoculars have their observational lenses set wider than the ocular lenses which line up with the eyes. These tend to be bulky and take up considerable room in bags or when strung around the neck. Their larger observational lenses, usually 40mm-50mm, receive more light than compact binoculars, and for work in low light conditions such as twilight or in dark woodland, they are a superior instrument. Their shape accommodates Porro prisms, a staggered dog-leg arrangement for bouncing the received light. Porro prisms usually manipulate the light path through mechanical movement of the eyepieces in or out to focus the binoculars and can be prone to premature wear or resultant misalignment in poorly manufactured equipment.
Straight barrelled binoculars that lie parallel to each other utilise a roof prism system to bend light, so the image appears erect in alignment with the eye. Roof prisms are technically more difficult to produce, but they provide superior results with their slimmer profile. In this style of binoculars, both the ocular lens (the part closest to the eyes) and the observational lens are in line. This has no effect on exit pupil or magnification capabilities.
Compact binoculars refer to a range of observational lens sizes with the effective diameter measuring less than 25mm. Smaller lenses are carried in smaller barrels, so weight is considerably reduced. Mid-range lenses of 32mm diameter offer a reasonable compromise for all-purpose observation without adding extra bulk to the binoculars.
Contoured grips and rounded edges make carrying binoculars more comfortable. Hinged barrels fold against each other for convenient storage.
Understanding Exit Pupil and Relative Brightness
A set of binoculars 10x25 has an exit pupil of 2.5mm (25/10), and a set 8x30 has an exit pupil of 3.75mm (30/8). The calculation to work out exit pupil size so comparisons can be made between models is: objective lens effective diameter/ magnification. In the example above, it shows that the binoculars with the smaller magnification value (8) have a better exit pupil size (3.75) because of the larger objective lens (30mm).
Relative brightness is a good indicator of how well a pair of binoculars can offer sharp, well-defined images. It is calculated by squaring the exit pupil size. Using the 8x30 3.75mm example, the relative brightness would be (30/8)2 or 3.75 x 3.75 = 14.06. For the 10x25 2.5mm example, the relative brightness would be (25/10)2 or 2.5 x 2.5 = 6.25. If instead the magnification is the same and the lens size is the only difference, binoculars with the same magnification will experience brighter lit images when the objective lens is larger.
Despite mathematical calculations which help prove a theory, in reality the quality of optical glass and coating/s used in binoculars will affect the actual brightness values as a visual experience.
Keep a Steady Hand
Up to 10x magnification binoculars can be hand held and the image is not too badly affected by hand shake. Once magnification values go beyond this level, all visual noise is exaggerated and can interfere with the viewed image. This is overcome by stabilising the binoculars on a tripod so they are fixed and secure.
Observing Objects Close-Up
Not every binocular user wishes to observe distant objects. When magnification is required for seeing close focus detail of small objects, the close focus function needs to be short. The minimum close focus distance for binoculars is within a 1m-2m range, which permits appreciation of fine detail on paintings or small scientific specimens.
Eye relief is the factor most influential on eye comfort. For the best image, a viewer should line up their pupils with the exit pupils of the binoculars (adjusted if required) as close as possible to the ocular lens creating the eyepoint. This way they can appreciate the whole view field. The distance between the lens and the eyepoint is known as the eye relief. High eyepoint binoculars have eye relief of 15mm or more, which makes them more suitable for spectacle-wearers as their spectacle lenses create distance which non-spectacle-wearers do not experience. A shorter eye relief, though acceptable for those not wearing glasses, may cause vignetting for spectacle wearers unless they remove their glasses.
How to Find Binoculars on eBay
To get to the best matches for binoculars on the eBay site, start at the home page and let the cursor hang over Electronics & Technology to reveal more categories, then choose Cameras & Photography. Within this category, select Telescopes & Binoculars, and on the new page click on Binoculars. All the current listings in the Binoculars category will appear, so if a specific brand or price limit is a concern, check the boxes to suit the search. Another means is to type 'Binoculars' into the main search box; click Go, and the site will return as many matches as possible throughout the categories.
Choosing a pair of binoculars is a personal decision. Take the opportunity to test drive some in retail outlets and get a feel for the size, weight, and comfort of different models. This will help refine any search conducted on eBay for the make and model that suits best. Back up the test drive with some research to compare performance, quality, and price between models.