My notes says the following things about diffraction:
The larger the ratio of wavelength to slit (or object) width, the more pronounced the diffraction is and the more spread out the wave energy is. Diffraction is most obvious when the wavelength and the dimensions of the obstacle/space are the same order of magnitude (approximately equal in size).
I find the second statement contradictory to the first statement. If diffraction is 'most pronounced' when $\frac{\lambda}{d}$ is the greatest, why is it 'most obvious' when $\frac{\lambda}{d} \approx 1$? (I interpreted the statement 'the wavelength and the dimensions of the obstacle/space are the same order of magnitude (approximately equal in size)' as $\frac{\lambda}{d} \approx 1$, not sure if this is correct.)