focal length (redirected from Back focal distance)

length (l ) (length) the longest dimension of an object, or of the measurement between the two ends.

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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 persons.

focal length  the distance between a lens and an object from which all rays of light are brought to a focus.

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focal length

n.

The distance from the surface of a lens or mirror to its focal point. Also called focal distance, focus.

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focal

1. limited to a small area or volume.

2. pertaining to or emanating from focus.

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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.

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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

f = r/2

Syn. focal distance. See principal focus; mirror; cardinal points; principal points; equivalent power; refractive power; sign convention.