Image acquiring apparatus and image acquiring method for detecting and recognizing size of scan target without using sensors

An image acquiring apparatus and image acquiring method are provided. The image acquiring apparatus includes an image sensor for outputting a plurality of first analog image signals corresponding to a plurality of pixels; an ADC for converting the plurality of first analog image signals to a plurality of digital image signals; a DAC for converting the plurality of digital image signals to a plurality of second analog image signals, wherein each of which has an actual voltage value; a comparing unit for respectively comparing the actual voltage values of the plurality of second analog image signals with a reference voltage value so as to generate a plurality of corresponding comparing results; a counting unit for adjusting a counting value according to each of the comparing results; and a control unit for determining whether to stop a scanning operation of the image acquiring apparatus or not according to the counting value.