1. The point at which rays of light converge after passing through a convex lens to form a real image (
real focus), or diverge from (
virtual focus) after passing through a concave lens.
2. The centre or starting point of a disease process.
3. To adjust an optical system (e.g. camera or projector) in order to obtain a sharp image.
Plural: foci.
Syn. focusing.
See confocal;
principal focus;
focal line.
aplanatic foci A pair of conjugate object and image points for which an optical system is free of spherical aberration.
Syn. aplanatic points.
dark focus See resting state of accommodation.
depth of focus See depth of focus.
principal focus The axial image point produced by an optical system of an infinitely distant object (the
second principal focus or
posterior principal focus), or that axial object point for which the image will be formed at infinity (the
first principal focus or
anterior principal focus). A converging optical system or lens has two principal foci that are real. A diverging optical system or lens has a second principal focus that is virtual. In curved mirrors the two principal foci coincide. Depending upon whether the object is at infinity or at the principal focus, this same focal point becomes either the second principal focus or the first principal focus, respectively.
Syn. focal point.
See focal length;
equivalent power;
sign convention.
real focus See focus.
sagittal focus; tangential focus See oblique astigmatism.
virtual focus See focus.