Applying frequency-domain filters on a centered Fourier transform
Note that an FFT of strictly real data is conjugate symmetric.  

For a length M FFT of strictly real data, the data in FFT result bin M-1 has the same magnitude as in bin 1 (for a low frequency), but complex conjugated.  So to maintain symmetry after filtering, which is necessary for the IFFT-ed image to remain strictly real, a filter has to modify bin(M-i) by the same ratio as bin(i).