focus

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focussearch for term: 1) To bring beams of light into a convergent point, or focal point. An image created by beams of light at the focal point is said to be "in focus." Focused images have sharp edges, as opposed to being fuzzy or blurry. In smaller telescopes, astronomers place an eyepiece close to the focus in order to project the telescope focal point onto an observers eye. In larger telescopes, astronomers place a CCD camera at the focal point to capture the focused light.

2) One of two fixed points inside an ellipse. The foci of an ellipse help to define the curve of the ellipse itself.