focal length(redirected from Back focal distance)
Also found in: Dictionary, Thesaurus, Acronyms, Encyclopedia.
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.
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.
In optics, the length from the lens to the point of focus of light rays passing through the lens.
See also: length
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
f= 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.
see focal-film distance.
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.
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.
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.