Was reading this answer (emphasis mine):
It also tells us that the lens was stopped down, as if it were wide open there would be no corners to cause diffraction, regardless of the number of aperture blades.
What does "stopped down" mean?
Answer
This picture shows a lens at different apertures. I used a manual focus lens (a Soviet-built 50mm f/1.9) because having aperture controls on the lens itself made it easier to take pictures of the lens at different apertures.
In the top third, the lens is wide open, or at its maximum (widest) aperture (lowest f-stop value), in this case f/1.9. The aperture blades are fully withdrawn into the lens housing and do not restrict any light from passing through the lens.
In the bottom right, the lens has been stopped down about one stop to f/2.8. It is no longer at its widest aperture: you can see the aperture blades have physically extended into the lens opening slightly, to restrict some of the light coming through the lens (if I understand correctly, one stop cuts the light coming through the lens by half; this lens has been stopped down by just over one stop (from f/1.9 to f/2.8; normally one stop would be from f/2.0 to f/2.8) so the aperture blades should actually be covering just a little more than half of the area of the lens).
In the bottom left, the lens has been stopped all the way down to f/16, it's smallest aperture (and highest f-stop value). This lens has six aperture blades; in this portion of the picture you can see three of them meeting.
(I apologize for the different exposure of the bottom-left photo. I was shooting in Av, not Manual; perhaps the different reflection of light from the fully-stopped-down lens convinced my camera to up its exposure by 1/3 stop. I'll leave this as a reminder to readers to shoot in manual whenever you're combining multiple photos)
Here is a picture of the lens at f/16 to see what it looks fully stopped down:
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