Hi,
Have you seen the example given in this link
https://www.mathworks.com/help/images/ref/psf2otf.html
PSF = fspecial('gaussian',13,1);
OTF = psf2otf(PSF,[31 31]); % PSF --> OTF
subplot(1,2,1); surf(PSF); title('PSF');
axis square; axis tight
subplot(1,2,2); surf(abs(OTF)); title('Corresponding |OTF|');
axis square; axis tight