1. A calibrating method for a printer, the calibrating method comprising: printing a calibration pattern on a medium by the printer, wherein the calibration pattern is an image filled with pixels each having a same, preset image level; obtaining a scanned image of the calibration pattern on the medium printed by the printer; comparing the scanned image against the calibration pattern; detecting defects of the scanned image; and if the defects exceed an pre-determined value, adjusting print settings of the printer according to the defects of the scanned image; wherein the step of detecting defects of the scanned image comprises: generating an image level distribution of the scanned image having a total number of pixels; deriving a reference image level which has a maximal number of pixels from the image level distribution; counting up a first number of pixels which have image levels outside a pre-determined range of the reference image level; and calculating a first proportion of the first number relative to the total number of pixels of the scanned image.