What is Modulation Transfer Function?
Imaging is simply a matter of contrasts. Any pattern consists of contrasts, usually with sharp edges. When contrast lines in the picture are clear, the image is perceived as sharp, while fuzzy contrast lines make the image look blurry. However, our eyes also tend to recognize sharpness by the level of contrast. Fuzzy lines with high contrast may appear sharper than fine lines with low contrast. Therefore, lens manufacturers try to optimize both criteria, high resolving power and high contrast. The limit for a sensor’s ability to distinguish contrast lines is fairly easy to understand. Any given pixel on the sensor can only define one gray level at a time. Therefore, a minimum of two pixels is necessary to define a contrast edge (one dark and one bright pixel next to each other), hence defined as “line pairs”. The restriction is the distance between two adjacent pixels. This fact is described as the “Nyquist theory”. For instance, the Nyquist frequency of an APS size sensor (22.5×15 mm)
