Use imshow() to see the spectrum.
I'm not sure who wrote your hint, but it's misleading. I could be wrong but with a Ph.D. in optics and 37 years of developing imaging algorithms I hope I know at least 0.001% of the imaging concepts out there and a pretty darn good intuitive feel for fourier domain concepts. The first clause of the hint talks about the power spectrum, but what it says after that applies to the SPATIAL domain, not the spectral domain. The slope does get flatter in the spatial domain, like when sharp steep edges get blurred out. It's not right to say that flat or steep slopes in the spectral domain necessarily mean anything. What's more important is the values out in the higher frequencies of the spectral domain. For example, if you have a periodic texture in the spatial domain, you would have steep slopes in some places in the spectral domain because the periodic structure in the spatial domain will give periodic spikes in the spectral domain. Now the author may be thinking of MTF curves where the slope at mid-level frequencies may (or may not) get steeper as the blurring gets worse. Again, it's the value (height) or the amount of power in those frequencies that is the crucial thing, not the slope . As the Full Width Half Max moves inwards as the image becomes more blurred, the slope may get steeper (higher) but you may have a system that has a steeper slope out at a higher frequency and has less blur. I could draw some MTF curves to illustrate if you want.
