focal length

Definition: (1) For SIMPLE LENS: distance between centre of lens and sharp image projected by it. (2) For thick, or COMPLEX LENS systems: distance between rear principal point (or rear nodal point) and rear principal focus. (3) For MIRROR: distance between optical centre of mirror and image: is equal to half radius for spherical mirror; an ellipsoidal mirror has two foci. NB: Focal length always measured on optical axis and with object at infinity. * Focal length is effectively distance between point from which image of object appears to be projected to image itself, where object is at infinity: this point of projection may be in front of and behind lens itself as well as being inside it. * Focal length does not necessarily correspond to physical length of photographic lens. * It is fundamental characteristic of lens or optical system as it (a) determines image size relative to subject distance (b) determines relative aperture or f/number.

Related Terms: telephoto

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