conjugate foci noun (physics)

Two points such that rays of light diverging from either of them are brought to a focus at the other

conjugate mirrors plural noun

Mirrors set so that rays from the focus of one are reflected to that of the other

focus /fōˈkəs/

noun (pl foci /fōˈsī/ or foˈcuses)

1. A fixed point such that the distances of a point on a conic section from it and from the directrix have a constant ratio (geometry)
2. A point in which rays converge after reflection or refraction, or from which (virtual focus) they seem to diverge (optics)
3. Any central point
4. The point or region of greatest activity or interest
5. The point of origin (eg of an earthquake)
6. The position, or condition, of sharp definition of an image

transitive verb and intransitive verb (foˈcusing or foˈcussing; foˈcused or foˈcussed)

1. To bring or adjust to a focus
2. To adjust so as to get a sharp image of
3. To concentrate

fōˈcal adjective

Of, belonging or relating to a focus

focalizāˈtion or focalisāˈtion noun

fōˈcalize or fōˈcalise transitive verb (technical)

To focus

fōˈcally adverb

focimeter /fō-simˈi-tər/ noun

An instrument for measuring the focal length of a lens

foˈcusing noun

A psychotherapeutic technique involving self-awareness

focal distance noun

Focal length

focal infection noun

A localized bacterial infection, eg of a gland, etc

focal length noun

The distance between the centre of a lens and its focus

focal plane noun

The plane, at right angles to the principal axis of a lens, in which the image is formed

focal-plane shutter noun

A camera shutter in the form of a blind with a slot, which is pulled rapidly across, and as close as is practicable to, the film or plate, speed being varied by adjusting the width of the slot

focal point noun

1. Focus (optics)
2. A centre of attraction for some event or activity

focus fund noun

An investment fund containing shares in a relatively small number of companies

focus group noun

A small number of people assembled to discuss a topic, new product, etc so that researchers may benefit from their opinions

focusing cloth noun

A cloth thrown over a photographic camera and the operator's head and shoulders to exclude extraneous light when focusing

conjugate foci (optics)

Two points such that each is the focus for rays proceeding from the other

in focus

Placed or adjusted so as to secure distinct vision, or a sharp, definite image

principal focus (optics)

The focus for rays parallel to the axis