Celestron SkyMaster Gemini 2x40 Astronomy Binoculars

The magnification factor, also known as "power", is listed as the first number given in a binocular's name, like 7 x 50, 8 x 40, etc. In the first example, these binoculars magnify an image 7X more than if you were looking at that image with the "naked eye". When talking about zoom binoculars, the magnification is listed like this: 7-21 x 50. That means the binoculars have a zoom mechanism that takes the power from 7X to 21X as far as the eye can see. Please note that it doesn't matter whether there is spacing between the numbers or whether the "X" is upper- or lower-cased.

Don't always assume that more magnification is better! The more power, the harder it is to keep the image stable when hand-holding binoculars. Most people can comfortably hand-hold up to an 8X (most popular for birding), but after that, you may or may not need a tripod for a shake-free view...it depends on your own steadiness and how long you plan to view.

The aperture of a pair of binoculars (or any optical instrument) refers to the diameter of the main optics. Of course, binoculars have two main objectives that are the same size, but the number given for the aperture refers to just one. For instance, 8x50 binoculars have an aperture of 50 mm (and a magnification factor of 8), but unlike a telescope, they have two 50 mm objectives. The larger the aperture, the bigger the binoculars, but more importantly, a larger aperture draws in more light and lets you see more detail in shadow, shade, and other low light conditions.

Use the Binocular Series search filter if you want to check out a particular line of binoculars from a manufacturer. Once you have chosen the Binocular Series, you will see all the models in that series that we offer. A series of binoculars will be very similar in most respects, other than their magnification factor, aperture size and price.

Shopping for binoculars by body size is a great help if you already know you want a pair of compact (small) binoculars or you are looking for large aperture optics for astronomy. In general, the larger the body size the heavier the binoculars will be and the more likely you may need to mount them on a tripod when viewing for any length of time.

The weight of a binocular is a small consideration next to other more important factors like body size, aperture, and magnification, but it is still nice to know what to expect.

Believe it or not, sometimes super small, lightweight binoculars are harder to hand-hold without unwanted binocular movement than larger binoculars are...a little heft helps stabilize them. However, heavier binoculars can fatigue your arms when trying to hand-hold for longer periods of time. If you decide to choose a pair of binoculars by weight, I would use the "Goldilocks" rule; pick something that is not too heavy and not too light...but just right! Unless, of course, you plan to mount your binoculars on a tripod, monopod or other type of support, then the weight doesn’t matter.

Want to know how to tell the difference between Roof prism and Porro prism binoculars? Roof prism models look like two straight monoculars connected together in the middle, while the eyepieces are closer together than the main optics of Porro prism binoculars, angling the light path.

Most binoculars are either roof or porro prism designs, but there is also a third option: Reverse Porro Prism. This design is used for small compact binoculars where the eyepieces are more widely spaced than the main optics. The prisms are actually reversed so that the light path is altered for this configuration.

Like most things in life, which design you should choose is not cut-and-dried. Here are a few basic statements about Porro and Roof Prism designs that may help you:

• If you are looking for a high-quality binocular, you will need to spend more on a pair of Roof binoculars than equal quality Porro’s. Roof prisms are more expensive to make.

• Roof prism binoculars are more durable because the prisms are cemented together rather than mounted separately.

• Men with large hands sometimes prefer holding a Porro Prism design.

• Both designs are comparable in all aspects when choosing high-end binoculars.

Close focus is pretty self-explanatory, it is the shortest distance you can be away from your subject and still focus your binoculars, however, why you might concern yourself with close focus isn't as obvious. If the only thing you'll be using your binoculars for is viewing at a distance, then you may not see the value in paying for extra-close focus. However, if you can think of any reason why you might want to have your body far away but view things up-close, it is well worth paying for. Bird-watching, bug & reptile observations at a safe distance, reading the small (or faded) print on signs, you get the idea!

Most binoculars come with the ability to adjust focus. A central focus knob or rocker platform, which rocks back and forth like a teeter-totter, is used by many manufacturers for its ergonomic location and ease of use. Some binoculars have individual eye focus (IF) so that you can tweak focus for each eye separately. There are pluses and minuses to both, although CF, or Center Focus binoculars are much more popular. Lastly, you may find a pair or two of Focus-Free binoculars. With these models, the focus is set to infinity and you can't change it. It may be a good option for children but most adults prefer the ability to focus.

While there are certainly individuals who don't require a pair of waterproof binoculars, we recommend it for most people. If you will be out on the water, at the beach, in snow, or find yourself in situations where heavy dew is present, you should consider the protection waterproofing can offer. If a pair of binoculars is waterproof, it can withstand getting caught in the rain, dropped in a puddle or getting a beer or soda bath at the ball park. Waterproofing can also keep fine dust and sand from entering the optical system.

Keep in mind that "water resistant" is not the same as "waterproof". Most binoculars that are water resistant can get a little wet as long as you dry them off as soon as possible, but they will not keep liquid out if submerged or kept wet.

Most binoculars include optics that have been coated to some degree. Optical coatings increase and even out light transmission as well as enhance contrast. What an optical coating consists of is often proprietary, but you can usually see the coatings by holding the optics at an angle to display a green, blue or brownish tint to the glass. While the addition of coatings is far better than plain, uncoated glass, it is important to know how many optical surfaces are coated. The more, the better. Watch for these terms:

• Coated: This means at least one side of one major lens is coated.

• Fully-Coated: All sides of all lenses are coated, including the longer side of the binocular's prism.

• Multi-Coated: This term means that the binoculars are Fully-Coated, and on top of that, at least one side of a major lens element is coated with multiple layers of anti-reflective material.

• Fully Multi-Coated: What you want to see when looking for a good pair of binoculars. All sides of all elements are multi-coated, providing the best light transmission (up to about 95%), sharpness, and contrast. If the coatings are advertised as "Broadband", that is great. A broadband coating will increase light transmission across the entire spectrum.

Phase-coatings are often applied to binoculars with a roof prism (as opposed to Porro prisms) to help keep the light from going "out of phase" as it passes through the prism. If you decide to buy a pair of roof prism binoculars, watch for this specification, as it increases clarity, contrast and sharpness.

The size of the beam of light that hits your eye when you gaze through your binoculars or other optics is called the Exit Pupil. It is measured in millimeters. You can calculate the exit pupil by dividing the aperture by the magnification. Therefore, a pair of 8 x 50 binoculars would have an exit pupil of 6.25 mm (50 divided by 8). Basically, the idea is to match the exit pupil to the size of your own pupils. In low light, your pupils open up to let more light in. In bright light they get smaller. So binoculars with a larger exit pupil are good for all lighting conditions, while those with a small exit pupil do not perform as well in low light.

Eye relief is the minimum distance between your eye and the binocular’s eyepiece that allows the full field of view to be comfortably observed. The measurement is in millimeters and is taken from the last lens surface to the point beyond the eyepiece where all the rays of light come to focus and form a sharp image. Generally, eye relief should be at least 10 mm, but you will find that about 15 mm will provide best comfort without glasses. If you have an astigmatism and need to wear glasses when observing, you may require 15 mm - 20 mm of eye relief for comfortable viewing.

Pupillary Distance is the distance between the pupils of your eyes, measured from center to center in millimeters. Most binoculars have hinges that allow the distance between the eyepieces to expand or contract in order to match the proper spacing for your eyes. Minimum Inter-pupillary distance, therefore, is the measurement in mm from the center of the left eyepiece to the center of the right eyepiece when the binoculars are "folded in" as far as possible.

Pupillary Distance is the distance between the pupils of your eyes, measured from center to center in millimeters. Most binoculars have hinges that allow the distance between the eyepieces to expand or contract in order to match the proper spacing for your eyes. Maximum Inter-pupillary distance, therefore, is the measurement in mm from the center of the left eyepiece to the center of the right eyepiece when the binoculars are spread as far apart as possible.

Angular field of view is the true angle seen through binoculars and is usually measured in degrees. Linear field of view, on the other hand, is usually measured in feet observed at 1000 yards, i.e. 420 ft. @ 1000 yards. High Point displays the field of view given by the manufacturer. Most users prefer a wider field of view; it is especially helpful when observing moving objects, like when you're at a football game or riding in a boat or car.

Linear field of view is the width of the area you can see through your binoculars and is usually measured in feet observed at a distance of 1000 yards, i.e. 420 ft. @ 1000 yards. Angular field of view, on the other hand, is a measurement of the true angle as seen through the binoculars, but it is normally stated in degrees. High Point displays the field of view given by the manufacturer. Most users prefer a wider field of view; it is especially helpful when observing moving objects, like when you're at a football game or riding in a boat or car.

Most binoculars, with the except of very small models, are designed to be attached to a tripod with an optional binocular tripod adapter. Using a tripod when observing for longer periods of time or at high magnification (or both) helps keeps the image shake-free. If this filter says "No", but you would like to use a tripod, there are a handful of adapters that wrap around the binoculars instead of threading into a 1/4-20 built-in port.