What makes me wonder is the observation in step 5: why does constricting pupil lead to lessening of the diffraction smearing of the light sources? Shouldn't it behave similarly to photographic systems, where the smearing increases?

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Dilation of the pupil of the eye should not have a huge impact on ciliary corona, but if the pupil of the eye dilates, another diffraction phenomenon, which is brighter, may appear: lenticular halo.

Its probably the effect of switching from low intensity gray receptors to normal colour receptors, that don't register the spikes.

I think when the iris is wider you could be seeing diffraction from hairs on your eyelids (eyelashes), and when the iris is smaller it isn’t looking ‘past’ the hairs so you don’t see them. I’m just wondering why we don’t see diffraction softening as in cameras when using a small (bigger number) aperture

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With an open iris and higher overall intensity inside the eye by reflections, the signal of the gray level intensity cells are switched off by saturation. In the state of saturation the single photon detection by receptors does not work anymore. This is the effect, that makes is possible to see by starlight only even in the dark of the forest.

The radial diffraction spikes seen by normal human eyes around bright point sources are called ciliary corona. They can be seen when the apparent diameter of the point source is less than 20' (so the Sun is too big for producing ciliary corona, although kids like to add radial spikes when drawing the Sun). The diffraction spikes are due to thousands of small scatterers in the eye lens, protein aggregates with a size of about 1 μm. You can simulate the ciliary corona by looking through a glass powdered with fine lycopodium powder.

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This happens due to diffraction on the aperture, which has a polygonal shape when closed. The smaller the aperture, the wider the spikes. This is easy to understand, because this is the case of Fraunhofer diffraction, where the far field is basically the Fourier transform of the aperture indicator function.

I can sometimes consciously play with the width of my pupils in the dark (this is an elusive ability, due to the indirect nature of control over iris muscles), and the effect is exactly the same, which confirms that this is not due to veiling glare or similar masking effects.

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