A photographic lens where assembled directly by the photographers changing lens configurations was converted in a tele lens for landscape or inverted for macro or camera was used as a projector to print negatives. History of lens design is history of photography. Modern science. Positive negative galilean telescope. Positive positive modern telescope. Ray from a star. Focus. Sensor or film. A very simple lens focusing an object at infinite distance. The object is in focus when sensor film is in the focal plane of the lens. Rays from a closer object. Focus. Sensor film. If the object is closer to have the image in focus I need to have a distance longer than the focal length. THAT IS FOCUSING.

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