At the top of the figure at which you were just looking you saw that to produce the image of a square wave edge it was combined with the spread function.
In the next line you see how the luminance of the image varies when the input to the lens is a square wave grating. The period of the grating and that of the image luminance is the same.
In the next line, the spatial frequency of the grating is increased. It would appear in this example that the hypothetical lens through which this grating is projected does not do as good a job of processing this higher frequency grating as it did the lower frequency. One comes to this conclusion because the amplitude of the image luminance wave has been reduced. In the last line the spatial frequency of the grating is increased again and the luminance of the image is even smaller.
Recall that the MTF is defined as the ratio modulations:
Click on low pass MTF to see a typical function.
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