A camera using an automatically depth-of-field adjusting method includes a camera lens, a diaphragm, a light-sensing part, and a controller. The light-sensing part can receive visible light of a first wave band and invisible light of a second wave band. The controller performs an adjusting procedure, according to which the controller measures a first light intensity of ambient light of a scene relative to the first wave band and a second light intensity of the ambient light relative to the second wave band, and when the controller determines that a ratio of a difference between the first intensity and the second intensity to a sum of the first intensity and the second intensity is less than a predetermined value, controls the diaphragm to adjust an aperture of the diaphragm according to a current focal length of the camera lens.