focal length



an expression of the longest dimension of an object, or of the measurement between its two ends.
crown-heel length (CHL) the distance from the crown of the head to the heel in embryos, fetuses, and infants; the equivalent of standing height in older persons.
Measuring an infant's crown-heel length. From Lammon et al., 1996.
crown-rump length (CRL) the distance from the crown of the head to the breech in embryos, fetuses, and infants; the equivalent of sitting height in older subjects.
cycle length in cardiac pacing terminology, the time interval in milliseconds from one event to the next; it is the inverse of the intrinsic rate (beats per minute) or the paced rate (pulses per minute).
focal length the distance between a lens and an object from which all rays of light are brought to a focus.

focal length

In optics, the length from the lens to the point of focus of light rays passing through the lens.
See also: length

length, focal

The linear distance separating the principal focal point (or focus) of an optical system from a point of reference (e.g. vertex, principal point, nodal point). The first (anterior) focal length is the distance from the lens (or first principal point) to the first principal focus. The second (posterior) focal length is the distance from the lens (or second principal point) to the second principal focus. Symbol: f.In a spherical mirror the focal length f, i.e. the distance between the focal point and the pole of the mirror, is equal to half its radius of curvature r
= r/2Syn. focal distance. See principal focus; mirror; cardinal points; principal points; equivalent power; refractive power; sign convention.


1. limited to a small area or volume.
2. pertaining to or emanating from focus.

focal distance
see focal-film distance.
focal length
the distance between the lens and an object from which all rays of light are brought to a focus.
focal liver disease
widely disseminated micro-abscesses or abscesses, migration paths of helminth larvae.
focal macular melanosis
dark colored spots on the skin, not elevated above the skin surface but apt to be confused with melanosis.
focal myelitis-encephalitis
widely disseminated inflammatory lesions, usually the result of blood-borne infection, in brain and spinal cord.
focal symmetrical encephalomalacia
see focal symmetrical encephalomalacia.
focal symmetrical spinal poliomalacia
see focal symmetrical spinal poliomalacia.
focal ulcerative dermatitis
manifest by ulcers in the unfeathered anterior breast skin of 4 to 5 months old male turkeys. The incidence is influenced by litter type. Called also breast buttons.
focal zone
in ultrasound, the area of the transducer where the sound beam is most sharply focussed and where the area under examination will give the best image.
References in periodicals archive ?
Simply, the wider focal lengths shorter than the 50mm 'standard' lens (the common starting point for many photographers), provide a field of view similar to most of our own visionary experiences.
To have the option of taking wide-angle and telephoto pictures, you needed other lenses of fixed focal lengths.
This year, Zeiss made the longest telephoto lens--1700mm in focal length and weighing 250kg
Alternatively, lenslets for such an array could be made with a much lower numerical aperture (which automatically reduces their optical aberrations) while still having a focal length much smaller than would be possible with macroscopic lenses.
Also it's not the focal ratio of a fast refractor that permits "wide-field views of the Sun" but rather its focal length.
Manufacturer Celestron Model C8-S Price (US$) $1,059 Mount included Optics type Schmidt-Cassegrain Aperture 203 mm Focal length f/ratio 2,032 mm f/10 Finder 6x 30 Mount type German equatorial Motor drive optional Computerized "Go To" no Also available: C8-SGT "Go To" system for $1,359.
This produces a rounded lens with a small focal length, so the drop can focus light only to a point near itself.
of Englewood Cliffs, New Jersey, provides a focal length of 7.
5180mm focal length range that can be ideal for a wide variety of studio productions that include news programs and variety shows as well as major events, supporting the emerging proliferation of 4K UHD content.
Using the same units of measurement, a telescope's focal length divided by an eyepiece's focal length gives the telescope's magnification.
The focal length of the objective is, more or less, the distance from the objective to the image that it forms.