Another tertiary effect that photographers are aware of is something called diffraction. This occurs when you have a very small aperture—the light is actually diffracted by the iris in a way that can be seen in the photograph. Generally, this effect is not desirable since it affects all the light in the image—nothing is perfectly sharp.
Here is a photograph that was set up to demonstrate the subtle effect of diffraction. It was taken at several apertures, and the subject is a bit of US currency.
It is a little hard to see, but the images have peak sharpness around f/8 and they get slightly less sharp to the right.
Next: Starburst