Lens Flare and Diffraction

Lens flare is a result of light refracting off the surface of a lens, it shows up as artefacts on your images, often as blue, green or red circles, low contrast or hazy areas and it can be used to creative effect. It is harder to achieve with newer lenses which have anti-reflective coatings, but can be achieved when there is a bright light source at the edge of the lens.

Diffraction causes the Starburst effect, seen on bright points of light when the Aperture is small, the light diffracts off the narrow gaps between the aperture blades. Again it can be used to creative effect.