focal length

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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 ?
Because I had no other choice of focal length, I researched and tried to learn everything I could about focal length.
Twentieth-century refinements by Rudolf Konig and Chester Brandon of the original 1860 design place the interior surfaces almost in contact to minimize ghost reflections and employ an eye lens of shorter focal length than the field lens.
3: Experimental setup of Sony G camera with four lenses of different focal length and a chessboard pattern (1830 x 1330)
Then it is hard to go past the Sigma 24-35mm f/2 DG HSM Art Zoom (700 [pounds sterling]) which covers the traditional wide-angle focal lengths of 24mm, 28mm and 35mm with a constant f/2 maximum aperture.
The lens is housed inside the main body and it comes out quite a long way when zoomed to the maximum focal length.
But as recently as 20 years ago the standard lens supplied with a 35mm camera had a fixed focal length of 50mm, which captured the scene being photographed pretty much as the eye saw it.
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.
This market concerns the acquisition of 2/3-inch uhd lenses, All types of focal lengths (current focal lengths of at least 12 mm and long focal lengths of at least 50 mm), Accessories (remote control, Handles, Filters, Focal lengths, Etc.
The top-quality focusing mechanism moves smoothly without backlash, optimally supporting the photographer's creative interplay with the focal plane of high-speed fixed focal lengths.
When shooting close-ups of people, longer focal lengths are better because the will not distort your subject like a wide angle lens would.