| 摘要 |
A camera includes a viewfinder, an objective and a manually operated focus adjuster. An optical system located in the path of incoming image light splits the incoming image light beam into two component beams projected into respective first and second focal planes. First and second planar arrays of photosensitive elements are located in respective first and second photodetector planes. When the objective is correctly focussed and then defocussed in a first direction, the sharpness of the image on the first array increases and that on the other decreases; if the objective is defocussed in the opposite direction, the opposite occurs. A comparator has two outputs, connected to the elements of the first and second arrays via respective first and second circuit branches. Each circuit branch includes at least one subtractor producing an absolute-value output signal independent of the polarity of the difference between the input signals applied thereto. The subtractor inputs are connected to the outputs of respective elements of the respective one of the two arrays. The comparator output signal controls an indication, visible in the camera viewfinder, informing the user of the direction in which he should manually move the focus adjuster to improve the state of focus.
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