Diffraction in a lens is most prominent at the aperture and the size of the aperture determines the
amount of diffraction. When the aperture is small, diffraction is increased. When the aperture is
large, diffraction is decreased. As light traverses the aperture all of the waves that get through
will start to interact with each other. If the crest of a wave meets another crest, they will add
together to form a larger wave. Two troughs will add to make a bigger trough. A crest and a
trough will cancel each other out.
All of this "interference" is happening on a dizzying scale on the way to the camera detector. The
pattern that light forms is a well-defined pattern called a "diffraction pattern". The pattern is a
series of light and dark bands related to the interference of the waves. The center band of light
in the pattern is the most important and contains most of the light energy. The width of this center
band of light is related to the size of the aperture. Small aperture makes the band wider (more
diffraction) and a large aperture makes the band more narrow (less diffraction).
amount of diffraction. When the aperture is small, diffraction is increased. When the aperture is
large, diffraction is decreased. As light traverses the aperture all of the waves that get through
will start to interact with each other. If the crest of a wave meets another crest, they will add
together to form a larger wave. Two troughs will add to make a bigger trough. A crest and a
trough will cancel each other out.
All of this "interference" is happening on a dizzying scale on the way to the camera detector. The
pattern that light forms is a well-defined pattern called a "diffraction pattern". The pattern is a
series of light and dark bands related to the interference of the waves. The center band of light
in the pattern is the most important and contains most of the light energy. The width of this center
band of light is related to the size of the aperture. Small aperture makes the band wider (more
diffraction) and a large aperture makes the band more narrow (less diffraction).
This center band of light is termed the "Airy disc". The Airy disc is the smallest size that a point of
light can be focused on the detector and is a direct determinant of image resolution.
light can be focused on the detector and is a direct determinant of image resolution.
| An Airy disc (homemade, fairly accurate). The same as the previous image, but from above. |
Diffraction is a topic that you can make as simple or complicated as you wish. I will try to stick to
the basics. Diffraction is the apparent bending of light around an object. Diffraction is an
extremely important topic in macro photography as it is often the major determinant of image
resolution.
Diffraction doesn't really bend the light as it goes around an object. A good analogy to think
about is a wave coming through a gap in a seawall. The wave will hit the gap and it doesn't just
continue in a straight line, the wave will radiate out in all directions from the gap in a circular
pattern. Light acts in a very similar way.
the basics. Diffraction is the apparent bending of light around an object. Diffraction is an
extremely important topic in macro photography as it is often the major determinant of image
resolution.
Diffraction doesn't really bend the light as it goes around an object. A good analogy to think
about is a wave coming through a gap in a seawall. The wave will hit the gap and it doesn't just
continue in a straight line, the wave will radiate out in all directions from the gap in a circular
pattern. Light acts in a very similar way.
| Diffraction |
waves entering a gap without diffraction
| what really happens at the gap |
| A typical diffraction pattern associated with an aperture (seen from the side) |
