Contrast . Contrast can be defined as . ... Contrast Transfer Factor (CTF) is the ratio of modulation in the image at any spatial frequency to the ratio of the zero frequency. ... Blackwell and others defined the threshold contrasts required for visual perception. ... It is reasonable to apply the term modulation to the actual contrast in the image of the square wave. ... The contrast transfer factor for a square wave is always greater than that for a sinewave target of the same period. ...