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! 2D
| Point<br />spread<br />function
| Optical<br />transfer<br />function |-
! 1D
| Line<br />spread<br />function<br
| Modulation<br />transfer<br />function
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Often the contrast reduction is of most interest and the translation of the pattern can be ignored. The relative contrast is given by the absolute value of the optical transfer function, a function commonly referred to as the '''modulation transfer function''' ('''MTF'''). Its values indicate how much of the object's contrast is captured in the image as a function of spatial frequency. The MTF tends to decrease with increasing spatial frequency from 1 to 0 (at the diffraction limit); however, the function is often not [[monotonic]]. On the other hand, when also the pattern translation is important, the [[complex argument]] of the optical transfer function can be depicted as a second real-valued function, commonly referred to as the '''phase transfer function''' ('''PhTF'''). The complex-valued optical transfer function can be seen as a combination of these two real-valued functions:
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