The following code block shows how to plot the change in signal to noise ratio (SNR) with cutoff frequency (F) for LPF:

snr = signaltonoise(im, axis=None)
pylab.plot(ubs, snrs_lp, 'b.-')
pylab.plot(range(25), [snr]*25, 'r-')
pylab.xlabel('Cutoff Freqeuncy for LPF', size=20)
pylab.ylabel('SNR', size=20)
pylab.show()

The following screenshot shows how SNR of the output image decreases with the increase in the cut-off frequency of LPF. The red horizontal line indicates the original image's SNR, plotted for comparison: