The MTF, or Modulation Transfer Function, is defined as the ratio of the image contrast to the target contrast, expressed as a function of spatial frequency. That is, MTF(u) = C'(u) / C(u). C is the contrast in the target, C' is the corresponding contrast in the image.

The modulation transfer function (MTF) is the most widely accepted measure of the spatial resolution response of medical x-ray imaging detectors and has proved to be a valuable tool for determining and comparing detector performance.

The slanted-edge method specified in ISO Standard 12233 [5] measures the MTF of digital image acquisition devices by analyzing the image data of a simple knife-edge target captured by the device. The target is framing free, enabling precise focus by adjusting the shooting distance instead of the rough focus ring.

In most lenses, the contrast is higher at the center of the field of view than at the edges. For this reason, an MTF chart may also show curves in the sagittal and meridional directions. These curves illustrate how the contrast changes depending on the distance from the center of the image.