How to calculate lens cut off frequency

is lens cut off frequency the same as MTF?

Say I have a periodic bar pattern as input, distance 3f/2 away there is F/4 lens, and after distance 3f I should have the image.

my solution is: diffraction limit is 1.22*wavelength*3f/D=14.64wavlength, then on the image plane, I assume the image of two bars is 14.64*wavelength, then at object plane, the cloest distance is 7.32*wavelegnth.

This number is very high, assume wavelength is 0.5um, it is 300+ cycles/mm. but normal lens MTF is less than 100. Where did I do wrong?

Thanks

nobody help me on this? this is a question on our PhD qualify exam a couple years ago.