At Drifter Sister Discount Binocular and Telescope Store we believe in quality customer service, and this includes answering commonly asked questions posed by our customers. Below is a list of some commonly asked questions about optics. First up, though, are three important links regarding returns.

How do I return a defective product?
What is the return policy?
How do I contact Customer Service?

Now for the FAQs
The answers to the questions below are meant to be general in nature. See our Home Page for an opportunity to receive a FREE, in-depth report containing important information to help you decide which type of binocular or telescope is best for you.

Q. What do the numbers in a binocular model refer to?
One number refers to the magnification, a second number indicates the aperture of the binocular.
Magnification
The first number in a binocular's designation indicates how much larger, or closer, an image will appear. This magnification is often indicated with an "x", indicating the "power" of the binocular. An image will appear 10 times larger through a 10 x 50 binocular than when viewed without a binocular. Hand-held binoculars range from 6 to 10 power. Larger binoculars generally need to be mounted on a tripod.
Aperture
The second number of a binocular designation refers to the diameter of the objective lens (the larger end of the binocular). It is almost always directly related to the size of the binocular. The apertures of so called "giant binoculars", used mainly for astronomical purposes, may range from 70mm to 80mm, while compact models will usually range from 20mm to 25mm in diameter. A larger objective lens will gather more light and, theoretically, provide you with greater detail and a clearer image.

Q. What about wearing eyeglasses when using binoculars?
If you wear eyeglasses, you'll want a minimum of about 15mm of eye relief for comfortably viewing the entire field of view. Most modern binocular models have fold down rubber eyecups that allow you to bring your binoculars in close to your eyes. Extend these eyecups for comfortable viewing without glasses. The entire field of view will be visible when your eye is a certain distance away from the eyepiece.
Astigmatism - If you have moderate to severe astigmatism, it is a necessity to wear glasses for serious observing.
Far-sighted or Near-sighted? If your eye correction is for near or far-sightedness, then the binocular's focusing travel will usually accommodate for this and you won't need to wear the eyeglasses. However, while not optically necessary to keep the glasses on, it is much easier to do so. In the field, you use the naked eye to locate birds before lifting the binocs. Constantly taking your glasses on and off is not only frustrating, but it will slow you down on fast moving birds.

Q. What is long eye relief and why is it important?
The entire field of view will be visible when your eye is a certain distance away from the eyepiece. This distance, or eye relief, can vary from as little as 5mm to as much as 23mm. Models designed to give long eye relief are sometimes called high eye point binoculars. Without this increased distance, the eye glass wearer will only see part of the field of view seen by those who don't wear glasses. Keep in mind that most long eye relief models will not have a particularly wide field of view. If you wear eyeglasses, you'll want binoculars with eye relief of 15mm or more. Most modern binocular models have fold-down rubber eyecups so that eyeglass wearers can bring their binoculars in close to the eyes. Extend them for comfortable viewing without glasses.

Q. What is field of view?
When looking at the circular view of a binocular, the field of view is the widest dimension you can see. It is mainly determined by the design of the eyepiece and not the objective lens size. As a general rule, the field of view will decrease as the magnification increases. For example, a 10 power binocular will have a smaller field of view than a 7 power binocular of the same design. For observing at close quarters in deep woods, scanning the sky for raptors or large flocks of migratory birds, or for picking up fast moving objects, a wide-field binocular is desirable. (Note: small objects can "get lost" amid the confusion of its surroundings when viewed through a wide-field binocular of 9 to 11 degrees).
Note: Wide-field binoculars typically don't have long eye relief. Their larger prisms and eyepieces add weight and bulk to a binocular and on many models there is a noticeable loss in sharpness at the edge of the field. People who need to wear eyeglasses when viewing will usually have difficulty seeing the complete field of view with wide-angle optics, consider binoculars with longer eye relief.

Q. What is Image Stabilization?
These specialized binoculars remove shaking from the image for crisp views at high powers without a tripod. Great for boaters and sports enthusiasts. For most people these won't replace your more conventional binos. However, for birders, hunters and astronomers they may fill a niche.

Q. What do Coatings do and what different types are there?
More light to your eye means brighter images for you. Special anti-reflection coatings are added to reduce the amount of light absorbed and reflected as it passes through each air-to-glass surface and inside the prism itself. Several layers of these multi-coatings significantly increase the light transmission. Anti-reflection coatings also reduce glare and allow you to see sharper images and more details, especially under low light conditions.

Q. What about light gathering ability?
Your eye is uniquely designed to gather more or less light as conditions change, your binocular is not. Since the light-gathering ability of a binocular is fixed, it is important to select a model that best meets your eye's need for light as viewing conditions grow darker. As we grow older, the maximum dilation of our eye pupil diamater slowly decreases from about 7mm or 8mm at age 20 to about 5mm at age 50. (Allow for variation in these figures due to hereditary or environmental factors). Higher-end binoculars use quality glass prisms made of costly, high density glass, such as the BaK-4 type, and provide clear, circular exit pupils. The glass quality also affects how clear the image appears across your viewing area. Quality glass delivers a "flat field" with clarity across the entire field of view.
The exit pupil and the twilight factor are indicators of how well you will see an image on a bright day, at twilight or at night.
Exit Pupil
You can see the exit pupil as a circular beam of light in the eyepiece when you hold the binocular at arm's length. The exit pupil is the magnified image in the eyepiece as it leaves the binocular to enter your eye. Its relationship to the size of your eye pupil is important. It is much easier to keep the smaller eye pupil centered in the larger exit pupil of a binocular.
Viewing in bright conditions, the eye pupil ranges from 2mm to 3mm.
Viewing at twilight, the eye pupil ranges from 4mm to 5mm. Consider 8 x 32 or 10 x 40 models to meet this standard.
Viewing in dark conditions, the eye pupil ranges from 6mm to 8mm. You'll want a binocular offering the largest exit pupil you can handle. A 7 x 50 binocular may be hand-held, but you may want to mount larger models on a tripod.
Twilight Factor
During daylight hours magnification will be the principal factor in image resolution. At night when the eye pupil is dilated, aperture size is the controlling factor. In twilight conditions both of these factors control resolution effectiveness. Twilight factor compares binocular performance under these conditions. Binoculars with a higher twilight factor will do a better job of resolving images under dim light conditions.

Q. What is a BaK number?
You'll often see binoculars with the BaK-4 number. This refers to barium crown glass. This fine, high density, glass eliminates internal light scattering and produces sharp images.