Click the control or press S to activate.
The Spike Sharpness control, part of the Primary Diffraction Spikes control group, allows you to make the Primary Spikes more fuzzy or crisp.
Higher Sharpness values make the Primary Spikes appear sharper.
Tip: An image will often look most natural if the spike sharpness is matched to the size of the highlights, though in some cases sharper spikes can give an image a feeling of higher resolution.