Optical Design & Quality of Binoculars

Binoculars use image-erecting prisms to provide correctly oriented images. The two basic designs used in optical systems are the Porro Prism (off-set barrel) and the Roof Prism (straight barrel). Another modification of the Porro design is the Reverse Porro Prism, commonly used in compact binoculars to reduce overall size. Porro prisms tend to be bulky and have been traditionally larger and heavier than equivalent roof prism binoculars. With recent modifications in optical design and production methods, modern Porro prism binoculars are much smaller and lighter in weight and may actually be lighter in weight than a roof prism of the same configuration. The Porro design is capable of delivering a wide field of view with excellent image sharpness. Also, they can usually be purchased at almost half the price of a comparable quality roof prism model. Roof prisms are generally smaller and more streamlined in size and able to tolerate rough treatment better because of their compact optical design. Most models have an internal type focusing mechanism which gives them a high degree of structural integrity, making them less susceptible to internal fogging and to potential dust and moisture entry. Their complex prism configuration and the precise tolerances required during manufacture make top quality roof prism binoculars significantly more expensive than those of Porro prism design.

The optical design of a particular binocular model is a compromise of several parameters to limit the amount of certain aberrations inherent in any optical system. The quality of glass used in the binocular lenses and prisms will determine the light transmittance, color fidelity and clarity of the image. The size and design of the prisms will affect sharpness, with quality optical glass delivering clarity from edge to edge of the field of view (sometimes called a "flat field"). Binoculars which use prisms made of costly, high density glass (such as the BaK-4 type) will provide clear, circular exit pupils.

Collimation is the mechanical alignment of the optical elements in a binocular. Both right and left hand optical axis must have proper orientation and location within the binocular barrel and must be parallel to each other in a quality optical system. High quality mechanical construction will ensure that the lenses and prism blocks maintain their correct alignment and provide years of comfortable viewing without headaches or eyestrain.

Anti-reflection coatings reduce light loss and glare due to reflection of light at air-to-glass surfaces. Besides increasing the light transmission, coatings also provide sharper and higher contrast images and aid in detailed identification of low contrast subjects in limited available light. Multilayer anti-reflection coatings (multicoatings) provide a significant increase in light transmission over single layer coatings and are used in all high quality optical systems.

These are among the most important factors concerned with the optical quality and construction of a binocular. A poor optical instrument will not reproduce the image clearly and with frequent use may cause eye strain, headaches and fatigue. By investing in high quality binoculars, you will benefit with years of comfortable and enjoyable viewing.

Thanks to our friends at Eagle Optics for providing this information!

See also:
Spotting Scopes—Tips for choosing the right spotting scope
Birding Tripods—Tips for choosing the right tripod for you binoculars and scopes

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