F-NUMBER
Also called the f-stop, focal ratio, f-ratio, or relative aperture, the F-number is the ratio of the lens's focal length to the pupil entrance diameter. To calculate the F-Number, you must divide the focal length of an optic by the diameter of the entrance pupil.
FIELD FLATTENER LENSES
These improve the sharpness of an image around the edges, lowering distortion by reducing the curvature of the field aberrations that can occur when focus is placed on the center of the field of view. Focusing on the center of the field of view causes the edges to go out of focus, or for the center to go out of focus when focusing on the images. Field flattener lenses produce sharper, clearer images even at the lens periphery, and are used in most high-end optics.
FIELD GLASS
A type of binocular that uses a second lens (instead of a set of prisms) to magnify an object. Field glasses are more durable than prism binoculars, although the magnification strength tops out at about 5x.
FIELD OF VIEW (FOV)
The maximum width of the areas that can be seen through the eyepiece of an optical instrument, as measured either angularly in degrees or linearly in feet or meters at specified distances. For example, a 10X binocular with a six-degree angular field has a linear FOV of 315 feet at 1,000 yards. For conversion purposes, one degree equals 52.36 feet at 1,000 yards.
FIELD STOP
An aperture located at the image plane of an optical instrument that determines the extent and shape of the field of view, which is usually round in viewing optics and either square or rectangular in cameras.
FIELD, APPARENT
The angular subtense of the FOV as it appears to the eye as the result of magnification. Apparent field is calculated by multiplying the true angular field by the magnification. Accordingly, a 10X binocular with a six-degree true field has a 60-degree apparent field.
FILTER
A screen that allows only certain colors to pass through it; a transparent material that separates colors of light.
FILTER, COLOUR
Glass filters, each of a specific color, which screw onto eyepiece barrels for enhancing lunar and planetary detail. Various color filters reduce other interfering or scattered wavelengths that blur certain wavelength-specific features. Red filters, for example, bring out Martian surface detail while green increases contrast of Jupiter's Red Spot. Also called planetary filters.
FILTER, LIGHT-POLLUTION
A filter that threads onto an eyepiece or rear cell of a Schmidt-Cassegrain that blocks wavelengths of light pollution sources such as mercury vapor and high-pressure sodium, but pass wavebands specific to deep-sky objects, such as hydrogen alpha, hydrogen beta, and oxygen III.
FILTER, MOON
A glass filter in an aluminum cell that threads onto an eyepiece barrel and reduces the Moon's glare so that it can be comfortably observed. Without the eye being overwhelmed by moonlight, more lunar detail becomes apparent.
FILTER, PLANETARY
Glass filters, each of a specific color, which screw onto eyepiece barrels for enhancing lunar and planetary detail. Various color filters reduce other interfering or scattered wavelengths that blur certain wavelength-specific features. Red filters, for example, bring out Martian surface detail while green increases contrast of Jupiter's Red Spot. Also called color filters.
FILTER, SOLAR
A glass filter that fits snugly over the aperture of a telescope and allows the photospheric surface of the sun - sunspots and solar faculae - to be observed comfortably and safely. A good solar filter blocks some 99.99% of the sun. Observing the sun without a solar filter may cause serious damage to the eye.
FILTER, VARIABLE-POLARIZING
Variable-polarizing filters act as dimmer switches to bright celestial objects, including the Moon or a planet. The filter, which threads onto 1.25" eyepiece barrels, consists of two pieces of polarized glass mounted in an aluminum cell that, depending on how much it is rotated, varies light transmission from 1% to 40%.
FINDER (OR FINDER SCOPE/FINDERSCOPE)
A small telescope, with a wide field of view, mounted on the main telescope tube to enable an observer to easily locate celestial objects, and place them within the field of view of the main telescope. Note: In the 'red dot finder scope', you see a LED red dot in the center of the finder's visual field, which helps to locate the target.
FLARE/ABERRATIONS
Flare is the scattering of non-image forming light within an optical system, which appears as stray reflections, ghost images or a veiling glare that obscures details. Aberrations are optical defects that degrade image quality. These include: Astigmatism, Chromatic Aberration, Coma, Curvature of Field, Distortion and Spherical Aberration.
FLINT GLASS
Flint glass is the other of the two types of optical glass used to manufacture achromatic lenses. Flint glass has a low Abbe number, a higher dispersion, and a higher index of refraction than crown glass. A concave lens of flint glass is often used alongside a crown glass convex lens to manufacture an achromatic doublet lens, because the two types of glass compensate each other; optical properties. This reduces chromatic aberration.
FLUORIDE GLASS
This is a class of non-oxide optical glasses, composed of the fluorides of various metals. Optical elements made from calcium fluoride are often used in telephoto lenses to correct color aberration. However, fluoride glasses are being replaced with the use of low dispersion glasses, because these glasses possess higher refraction index, better dimensional stability, and are less fragile.
FOCAL LENGTH
The distance between the principal focus of a lens or mirror and its optical center.
FOCAL PLANE
The plane where the image formed by the lens or lens system is in sharp focus. In a camera, the focal plane is the sensitized surface of the film.
FOCAL POINT
This is a point where the light rays from an image come sharply into view after passing through the binocular or scope.
FOCAL RATIO (F-RATIO)
Defined as f value. This is the focal length of a lens (or mirror) divided by its diameter. A focal ratio of 8 is written as f/8.
FOCUS
To adjust the eyepiece or objective of an optical instrument so that the image appears sharp and well defined to the observer. Changes in viewing distances usually require changes in focus.
FOCUSING RANGE
All binoculars or scopes have the ability to be focused for infinity. So a primary point of distinction between product models is the minimum focus range (see "Close Focus").
FOCUSSER
The mechanism which holds the eyepiece and allows adjustment for focussing the image.
FOLDED LIGHT PATH
A combination optical configuration using lenses and mirrors to create a total scope length much shorter than the total focal length of the system. This provides a compact design yielding long focal length performance.
FOR RIFLESCOPES ONLY MOUNTING LENGTH
The maximum length of a scope's main tube to which mounting rings can be attached. For easy mounting on full-length bolt-action rifles, this area should be at least 5 inches long.
FREQUENCY
The number of waves that pass a point in a given unit of time.
FULL SIZE BINOCULARS
In comparison with compact binoculars, full-size binoculars offer better light gathering ability because of a relatively larger objective lens. For example, a 10x42 binocular is a full-sized binocular, while a 10x25 binocular is considered as a compact binocular.
FULLY COATED OPTICS (FC)
All glass surfaces that have any vulnerability to air are coated.
FULLY MULTI-COATED (FMC)
All glass surfaces susceptible to air are multi-coated.
FULLY MULTI-COATED OPTICS
Binoculars or scopes that have fully multi-coated optics have multiple coatings on all air-to-glass surfaces. See also "Coated/Multi-Coated Glass".
FUSED QUARTZ
It is manufactured by fusing (melting) naturally occurring quartz crystals of high purity at approximately 2000 °C, using either an electrically heated furnace (electrically fused) or a gas/oxygen-fuelled furnace (flame fused). Fused quartz is normally transparent. The optical and thermal properties of fused quartz are superior to those of other types of glass due to its purity.
FUSED SILICA
Glass consisting of high-purity silica in amorphous (non-crystalline) form. It is different from traditional glasses in that no other ingredients are added to lower the melt temperature. Fused silica, therefore, has much higher working and melting temperatures than soda-lime or borosilicate glasses.
FWHM
Full width at half maximum. The width of the transmitting band of an optical filter, measured at 50% of the peak transmission.