MATLAB image transformation

f = zeros(30,30);
f(5:24,13:17) = 1;% generates a rectangular function
F0 = fft2(f);% two-dimensional Fourier transform
F2 = log(abs(F0));
F = fft2(f,256,256);% two-dimensional Fourier transform of zero padding operation
F1 = fftshift(F);% move the zero frequency to the center position
figure;
subplot(221);
imshow(f,'InitialMagnification','fit' );% Display f
subplot(222);
imshow(F2,[-1 5],'InitialMagnification','fit');% Determine the display range of the pixel value
subplot(223);
imshow(log(abs (F)),[-1 5]);% logarithmic display zero-filled transformed image
subplot(224);
imshow(log(abs(F1)),[-1 5]);% logarithmic display frequency shift After the image