In physics, focus is defined as that point in which the rays of light converge or meet; or diverge in the case when light is refracted or reflected. In optics, it can mean a number of things including:
? A lens? focal point
? A lens? or a telescope?s eyepiece?s focal length
? The condition by which an object being viewed through an optical system is seen; either being in or out of focus
? A device used in optical systems in order to adjust its focal length, thereby making an image clearer
An image is in focus when the light from the object converges on almost one single point in the image, while an object out of focus will have light from it not converging very well.
In astronomy, interest in foci is usually concentrated to telescope use. To get a good view of distant objects in the sky, a telescope must be properly focused. Focusing of telescopes is easy, and comes instinctively even to beginners.
Adjusting the focus of a telescope is usually done either by moving its eyepiece or its primary mirror. This can be done by turning the wheels of a geared system, called the rack and pinion; or by turning a screw knob on the back of a telescope.