Binoculars

The Galilean design has the advantage of presenting an erect image but has a narrow field of view and is not capable of very high magnification.

The Galilean design is also used in low magnification binocular surgical and jewelers' loupes because they can be very short and produce an upright image without extra or unusual erecting optics, reducing expense and overall weight.

In aprismatic binoculars with Keplerian optics (which were sometimes called "twin telescopes"), each tube has one or two additional lenses (relay lens) between the objective and the eyepiece.

The optical solutions of Porro and Abbe were theoretically sound, but the employed prism systems failed in practice primarily due to insufficient glass quality.

Porro prisms require typically within 10 arcminutes (⁠1/6⁠ of 1 degree) tolerances for alignment of their optical elements (collimation) at the factory.

Maintaining such tight production tolerances for the alignment of their optical elements by laser or interference (collimation) at an affordable price point is challenging.

Relevant differences in optical performance in the sub-high-quality price categories can still be observed with roof prism-type binoculars today because well-executed technical problem mitigation measures and narrow manufacturing tolerances remain difficult and cost-intensive.

[26] Given as the second number in a binocular description (e.g., 7×35, 10×50), the diameter of the objective lens determines the resolution (sharpness) and how much light can be gathered to form an image.

This ease of placement helps avoid, especially in large field of view binoculars, vignetting, which brings to the viewer an image with its borders darkened because the light from them is partially blocked, and it means that the image can be quickly found, which is important when looking at birds or game animals that move rapidly, or for a seafarer on the deck of a pitching vessel or observing from a moving vehicle.

Narrow exit pupil binoculars also may be fatiguing because the instrument must be held exactly in place in front of the eyes to provide a useful image.

The twilight factor for binoculars can be calculated by first multiplying the magnification by the objective lens diameter and then finding the square root of the result.

The twilight factor without knowing the accompanying more decisive exit pupil does not permit a practical determination of the low light capability of binoculars.

However, not related to the binoculars optical system, the user perceived practical depth of field or depth of acceptable view performance is also dependent on the accommodation ability (accommodation ability varies from person to person and decreases significantly with age) and light conditions dependent effective pupil size or diameter of the user's eyes.

There are "focus-free" or "fixed-focus" binoculars that have no focusing mechanism other than the eyepiece adjustments that are meant to be set for the user's eyes and left fixed.

Later steel and relatively light metals like aluminum and magnesium alloys were used, as well as polymers like (fibre-reinforced) polycarbonate and acrylonitrile butadiene styrene.

The housing can be rubber armored externally as outer covering to provide a non-slip gripping surface, absorption of undesired sounds and additional cushioning/protection against dents, scrapes, bumps and minor impacts.

The performance of an optical coating is dependent on the number of layers, manipulating their exact thickness and composition, and the refractive index difference between them.

In general, the outer coating layers have slightly lower index of refraction values and the layer thickness is adapted to the range of wavelengths in the visible spectrum to promote optimal destructive interference via reflection in the beams reflected from the interfaces, and constructive interference in the corresponding transmitted beams.

Determined by the optical properties of the lenses used and intended primary use of the binoculars, different coatings are preferred, to optimize light transmission dictated by the human eye luminous efficiency function variance.

Maximal light transmission around wavelengths of 555 nm (green) is important for obtaining optimal photopic vision using the eye cone cells for observation in well-lit conditions.

As a result, effective modern anti-reflective lens coatings consist of complex multi-layers and reflect only 0.25% or less to yield an image with maximum brightness and natural colors.

Depending on the coating, the character of the image seen in the binoculars under normal daylight can either look "warmer" or "colder" and appear either with higher or lower contrast.

Subject to the application, the coating is also optimized for maximum color fidelity through the visible spectrum, for example in the case of lenses specially designed for bird watching.

Birdwatching is a very popular hobby among nature and animal lovers; a binocular is their most basic tool because most human eyes cannot resolve sufficient detail to fully appreciate and/or study small birds.

For hunting binoculars optimized for observation in twilight, coatings are preferred that maximize light transmission in the wavelength range around 460-540 nm.

Further, binoculars designed for military usage may include a stadiametric reticle in one eyepiece in order to facilitate range estimation.

[85] Modern binoculars designed for military usage can also feature laser rangefinders, compasses, and data exchange interfaces to send measurements to other peripheral devices.

Hand held models will be 5× to 8× magnification, but with very large prism sets combined with eyepieces designed to give generous eye relief.

This optical combination prevents the image vignetting or going dark when the binoculars are pitching and vibrating relative to the viewer's eyes due to a vessel's motion.

High-power binoculars can sometimes show one or two cloud belts on the disk of Jupiter, if optics and observing conditions are sufficiently good.