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One of two popular methods of camera focusing systems, the other is active. Passive has the advantage over active because it isnt fooled by glass in between the camera and the subject. Its also not dependent on subject distance. It works by measuring the subjects contrast and as such its main downfall is when the subject has no contrast. In such situations, such as snow scene or low light, the lens will focus in and out and may never reach sharp focus. Fortunately many cameras with this system have manual override.