There is no specific weight that automatically says a set of binoculars is compact. But a prospective buyer should consider binoculars to be compact if he can wear them around his neck for at least an hour without feeling neck strain or if they can be easily stored in a large coat pocket.
Most top manufacturers have produced binoculars with "compact" in the title that have a weight ranging from eight to 20 ozs. and a height of approximately 4 inches. The best compact binoculars weigh approximately 12 ozs. but still have the magnification capability and large objective lens available in heavier and more powerful models. The lower the weight, the more likely a user will be able to view an object with minimal shaking and thus have a more stable, clear and accurate picture.
All binoculars will be categorized by a number ratio. Examples include: 8 x 25, 10 x 25, 10 x 40, 12 x 40 and 12 x 50. The first number indicates how much the object being viewed is magnified and the second indicates the diameter in millimeters of the objective lens. The size of this lens determines how much light is used (or enters the binoculars) to give the viewer a clearer, brighter picture. If a pair of binoculars has a ratio of 8 x 25, the image will appear to the viewer as if he is eight times closer to the object, and the diameter of the objective lens is 25mm.
Sometimes you will see a number ratio of 8-12 x 40. This indicates the binoculars have zoom capability, and the magnification can be adjusted from a minimum of eight times to a maximum of 12. Depending on your price range, you want compact binoculars that employ the highest magnification possible and have zoom capability so you can use them for a wider variety of situations.
Prisms and Glass Type
Modern binoculars are constructed using two types of prisms: porro and roof. A prism is the glass object inside the binoculars that produces an image for the viewer with a series of light/optical transmissions. You will know if binoculars use a porro prism because the weight will be closer to 20 ozs. and eyepieces are offset, rather than in a straight line, from the objective lenses.
Roof prism binoculars are smaller and thus lower in weight and run in a straight line with the objective lenses and eyepieces. These weigh closer to 12 ozs. Although a roof prism is more expensive to manufacture, binoculars with these prisms are also more rugged than those with porro prisms because of a more compact shape and a focus dial housed inside the body.
Depending on your price range, the best compact binoculars employ a roof prism and will be constructed of Bak4 optical glass.
The best compact binoculars must be 100 percent waterproof. Whether you spend $50 or $800, binoculars are an investment and should last a lifetime. Since they are primarily used outdoors, and there is always a possibility of wet weather, you don't want to risk your investment by getting moisture trapped inside and fogging your lenses.
Be sure to choose binoculars with a center focus feature, a horizontal wheel or sideways dial in the middle. Center focus binoculars will also have a diopter adjustment that will allow the user to adjust the focus for each side.
Don't choose binoculars listed as focus free. As the name implies, these binoculars will provide you with very little versatility or capability to clearly view the most simple object, even from a close distance such as 100 meters.
Binoculars have chemicals applied to the outside of the objective lenses to help make images brighter and clearer. The coatings help more light travel through the lens glass and give the viewer a clearer picture. There are three types: fully coated, multi-coated and fully multi-coated.
Fully coated binoculars, common in many older models, is the least efficient of the three coatings. Multi-coated is a step up and has multiple layers of these special chemicals, allowing much more light to pass through the lenses. The best of the three is fully multi-coated and produces the brightest image possible of a quality binocular with a lens coating.
Choose a set of compact binoculars with the following features:
Magnification capabilities between eight and 12, preferably with a zoom capability.
To keep the weight down, objective lenses should be no bigger than 30mm.
A roof prism made of Bak4 optical glass.
100 percent waterproof.
Center focus feature.
Objective lenses classified as fully multi-coated