Imaging device and autofocusing method

An imaging device that converts an optical image, which is captured via an optical system having a focusing function, into a video signal by means of an imaging system,

• the imaging device comprising:
• a high frequency component extractor that adjusts the video signal with a preset gain controlled according to an imaging condition and detects intensity of a specific signal in a certain frequency band, which includes at least a high frequency component and is generated in a focus state of the optical image captured via the optical system, from the adjusted video signal;
• a setting module that sets a variation range of changing a state of the optical system, based on at least information on a magnification change of the optical image caused by a change of the state of the optical system, where the variation range is used for detection of a state changing direction of the optical system or a focusing direction for focusing the optical image;
• a signal intensity variation detector that detects a variation in intensity of the specific signal in the set variation range a specified number of times while changing the state of the optical system in the set variation range;
• a focusing direction detector that detects the focusing direction, based on the detected variation in intensity of the specific signal;
• and
• an optical system controller that detects the intensity of the specific signal while changing the state of the optical system in the detected focusing direction, specifies a certain state of the optical system giving a peak intensity of the specific signal as the focus state of the optical system, and controls the optical system to the specified focus state.