Binocular Questions

Binocular Questions Module
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Binocular FAQs

Q: What do the numbers mean? 10x42 or 8x40

A: The numbers represent the magnification and the objective lens size of the binocular. The number before the "x" represents the magnification. A 10x42 binocular has 10 times magnification power. You will see everything 10 times closer. The 42 represents the objective lens size measured in millimeters. The objective lens are the lenses pointing toward your subject. A 10x42 has 10x magnification with a 42mm objective lens.

Q: How much magnification do I need?

A: Binoculars are an example of “more isn’t always better”. Having too much magnification can make holding the image steady very difficult and bring less enjoyment to your views. The most common of binocular magnifications are 8x or 10x magnification. The vast majority of binocular owners will select one of these two magnification powers. Binoculars can range in powers between 6x and some get as much as 25x or higher. The typical limit to comfortable views hand-holding a binocular is 12x, going over that can produce an image that is very unsteady. The balance that must be considered is a combination of:

1. Image stability. The more you magnify the close you get, at the price of a less stable image. 2. Field of view. The more you magnify, the narrower your field of view becomes 3. Image brightness. The more you magnify, the dimmer your image becomes (given that the objective lens is the same size)

It may be best to consider the options between 8x and 10x unless you specific activity requires otherwise. 8x will give you a brighter, more steady image with a larger field of view. 10x will get you a little close, still providing an image that can be hand-held with minimal sacrifice to image brightness.

Q: When is it ok to get binoculars with magnification higher than 8x or 10x

A: As long as you are aware that with greater magnification power, sacrifices in other important attributes arise. Many will mount high-powered binoculars on a tripod to eliminate hand-holding and provide a steady image. Some may be using binoculars of 15x or 20x for surveillance or quick spotting distance objects and not be concerned about the shake or narrow field of view. Some may be supplementing their everyday 8x binocular with another higher powered option.

Q: How should I decide what objective lens size is right for me?

A: Binocular objective lenses are measured in millimeters. An 8x40 binocular has a 40mm objective lens. An 8x25 binocular has a 25mm objective lens. Both binoculars see 8x magnification. The difference comes from the objective lens sizes which also determines the physical size of the binocular. A 40mm binocular is considered a full-size binocular, while a 25mm is considered a compact. A 40mm binocular can gather more light and provide brighter images. A 25mm binocular will have a dimmer image but be nice and compact. Start by deciding what is more important, compact and portability, or performance and light gathering.

Q: Will I notice a difference between compact binoculars and full size binoculars?

A: Yes. When magnification is the same, for example comparing 8x40 and 8x25, both will have the same magnification level. On bright sunny days or viewing objects that are in great lighting conditions you may hardly notice the difference. However, when viewing something in lower light situations – a bird up in a shaded tree area or a deer out in the brush – the difference will be extremely noticeable. The larger light gathering 40mm objective lens will provide a substantially brighter image. A common phrase among binocular enthusiasts is “the only thing good about a compact binocular is its size”.

Q: What makes binoculars have such a wide range of prices?

A: For the sake of keeping it simple – we will answer this in basics. Much more information on this can be found in further discussion. Many ask why is one 10x42 binocular 50 dollars, another one 300 dollars, and another 2000 dollars. Why is this when each binocular sees 10x magnification and each has the same 42mm objective lens size? It starts with the quality of glass used in the binocular. Some binoculars use exotic glass that maximize light transmission and image quality. Some use exotic lens coatings to continue to improve the quality of the image. And some binoculars have construction features like waterproof and nitrogen purged that can increase their costs.

Q: What would I notice different in a 300 dollar binocular vs a 2000 dollar binocular.

A: A beginner might not immediately notice any difference between those. 300 dollar binoculars produce an extremely nice image. But it would not take long at all comparing that 300 dollar binocular to the 2000 dollar model. You will begin to notice a brighter image, and an image that is “flat” across the entire field of view. You will notice colors are vibrant. You will notice increased contrast and brilliance. After viewing for an even longer period of time, you may notice less eye-fatigue. You will begin to see the very subtle differences that are brought out as you continue to climb up in binocular quality. 300 dollar binoculars are extremely nice! It can quickly cost a lot of money to get those small, but greatly enjoyable differences in your views through binoculars.

Q: Roof Prism binoculars or Porro Prism binoculars?

A: On your quest for the perfect binocular you will come across the options of Roof Prism binoculars and Porro Prism binoculars. The Porro Prism binocular has the familiar “dogleg” shape to the housing. The objective lenses are off-set from the eyepiece lenses. Roof Prism binoculars have an in-line shape to them so the eyepiece looks straight through to the objective. Roof prism binoculars are generally more compact than porro prism binoculars with the same objective lens diameter. When comparing binoculars of equal optical quality, the roof prism binocular will almost always be more expensive than the porro prism binocular. Roof prism binoculars have a much more complex system of optics than a porro prism, and that complexity comes at a significant higher price.

In the last 10 years, the top-manufacturers of binoculars have moved to the Roof Prism design. Do a search of the most expensive binoculars and you will see Roof prism binocular overwhelm the porro prism. While some of the least expensive binoculars may out-balance with porro prism designs. This can be confusing when many say that the very best binocular view is found in the highest of high-end porro prism models. These ultra-high end porro prism binoculars are simply not as common as the high-end roof prism design.

A porro prism binocular will be bulkier in size than a roof prism. Roof prism binoculars are “tougher” some say, as keeping them in collimation is easier than a porro prism binocular. Its fair to say that in today’s binocular market, the Roof Prism binocular has begun to take over.

Q: How do I focus a binocular?

A: This is a valid question because many first time binocular owners simply do not do it correctly. It’s not as simple as “turn the knob until the view looks good and sharp”. One of the most overlooked, and very important factors is the Diopter Adjustment. Nearly all binoculars will allow correction for people whose eyes are not the same. If one of your eyes is focusing slightly different than the other eye, than having a binocular focusing both lenses equally will never produce a perfectly sharp image. To correct this you use the binoculars Diopter Adjustment.

The diopter adjustment is found on many binoculars on the right eyepiece. Although some have it built into the main focus knob, its most common to find it on one of the eyepieces. This extra focusing ring will only adjust the focus of that single eyepiece – and will allow a negative or positive adjustment from the opposite eyepiece. To adjust your diopter setting follow these simple steps:

1. Look for an object that is beyond the close focusing distance of the binocular, but not too far out in the distance. 2. Look through the binocular, while closing your eye of the side that has the diopter adjustment 3. Keeping your eye closed on the side with the diopter adjustment, use the MAIN focusing adjustment to get your object sharp. 4. Next, without changing the main focus - open your eye of the diopter side and close your eye of the other side. 5. Use the DIOPTER focus to focus the open eye to the image. 6. You have now correctly compensated for any difference between your eyes. Return to viewing with both eyes and focus now using the main focus wheel. Leave the diopter focus where it is.

Q: What is field of view and how is it measured?

A: The field of view of a binocular is the distance you see across the image field. When viewing with the naked eye you have a wider field of vision than you do when magnified. Magnify 6x will narrow your field of view, and increasing to 8x will further decrease your field of view. The more you magnify the more you decrease your field of view.

A misconception about field of view is that its determined by the objective lens size. Or that the objective lens size is the field of view. Field of view has more to do with the physical length of the binocular and the configuration and design of the lens elements.

The common measurement of field of view for a binocular is “Feet at 1000 yards”, also known as True Field of View. This is often difficult to understand. For example, a binocular may have a “345 feet at 1000 yards” true field of view. To help understand this, imagine a fence that is infinitely long from left to right and is 1000 yards away from you in the distance. When viewing out of a binocular with a 345 ft. field of view, you will see 345 feet of that fence from left to right. Binoculars field of view is also measured in degrees and is known as Apparent Field of View. A binocular with a 6.6 Degree field of view will have a 345 foot linear field of view.

You can convert from one measurement to the other by knowing that one angular degree is equal to 52.5 feet. So if you know the angular field of view in degrees, multiply that number by 52.5 will give you the linear field of view of feet at 1000 yards. Or you can divide the linear field of view by 52.5 to get the angular measurement.

Q: What is eye-relief and what is important about it?

A: Eye relief is the distance from the outer surface of the eyepiece lens to your eye. This distance is measured in millimeters. When viewing through a binocular the eyecups are designed to hold the binoculars at the correct distance from your eyes. This is not important when not wearing eyeglasses as its simple to get to the proper position for the best view. When eyeglasses come into play, having a long eye relief becomes an important factor. If you wear eyeglasses then you will need to turn down the eye cups to match the distance your eyes are from your eyeglass lenses to the lenses in the binoculars eyepiece. This will help get you to the “sweet spot” and produce the maximum viewing image.

Q: What is exit pupil?

A: The exit pupil of a binocular is the magnified image in the eyepiece as it leaves the binocular to enter your eye. Its diameter is measured in millimeters and is easily calculated on all binoculars by dividing the objective lens size by the magnification. For example a 10x42 binocular has an exit pupil of 4.2mm and an 8x50 binocular has an exit pupil of 6.25mm. You can actually see a binoculars exit pupil by simply holding the binoculars at arm’s length and looking at the eyepieces. You will see the two small “holes” that the image comes through.

Exit pupil determines the brightness of the binocular. The connection is between your binoculars exit pupil and how it relates to the pupil size of your eye. Your eye pupil is controlled by the iris which acts like a variable aperture for the retina and allows the pupil to change in size from about 2mm up to 8mm. This depends on the brightness of the available light.

On a sunny day when viewing through a 8x20mm binocular with an exit pupil of 2.5mm, it will appear just as bright as a 7x50 binocular with an exit pupil of 7.1mm. Your eye may only adjust its pupil to 2 to 3mm in diameter during sunny lighting conditions. The extra light coming in from a larger exit pupil binocular on a daytime adjusted pupil will not be noticed by the eye. Its in low-light conditions where exit pupil becomes important to viewing through binoculars. In shadows and poorly lit areas of viewing your pupil size may be larger at about 4-5mm. Allowing all available light to come through the binocular to get into your eye for viewing.

Q: What is close focus?

A: Close focus is the measurement of the nearest distance you can focus a binocular. This is commonly measured in feet. Close focusing is useful for certain binocular activities such as butterfly watching, viewing insects, looking at wild flowers or even viewing paintings up close at a museum. A binocular with a close focus between 4 and 8 feet are considered good for close focusing.

Q: What is interpupilary distance?

A: The interpupilary distance (IPD) is the distance between the centers of the left and right exit pupil of a binocular. It’s the amount of adjustment that can be set for the binoculars spacing between your eyes. This is adjusted on a binocular at the “hinge” that allows the spacing of the eyepieces to be adjusted. Adjusting this properly will help to produce a better viewing image without the shadowing that can happen when not properly set. Finding the sweet spot for you is very simple and happens very naturally. The overwhelming majority of people will fit correctly on nearly any binocular with standard binocular adjustment. Some people with eyes very narrow or further apart than average may want to seek a binocular with an larger adjustment range of interpupilary distance.
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