Take a look at my post here: http://www.mathworks.com/matlabcentral/answers/24965-fft2-function-in-matlab
The example I show assumes that if you take the 2D Fourier transform of an MxN matrix that both M and N are divisible by two. If either M or N are odd, then the definitions of the frequency ranges will have to be defined differently. Around the part where I start plotting stuff, I compute the 2D FFT using Matlabs fft2 and I use fftshift to be able to plot it according to the frequency ranges I define earlier.
Let me know if you have any questions.
