A lens flare is a light artifact in shots which is a result of light entering the lens. The appearance of the lens flare depends on your lens diaphragm but usually it's a circular-type shape at its centre, with streaks emanating out of it.Usually lens flares are undesired artifacts which dilute the contrast of the shot, but sometimes they can be used to enhance a photo.
How do lens flares form?
To understand how lens flares form, first you need to understand that there are two main types of light:
1.) Image light: These are light rays which bounce off the objects within the image you are trying to capture.
2.) Non-image light (aka "stray light"): These are light rays which are entering the camera from places outside your intended shot.
When non-image forming light enters your camera lens, it can create a lens flare through the internal reflection of stray light in the lens.
The most common non-image forming light is from the sun being in your shot i.e. pointing your camera onto a scene that has the sun within your viewfinder, or just outside it. Aside from the sun, lens flares may be the result of other very bright objects shining light into the shot or just outside it, including light bulbs, the moon, and reflective surfaces.
Not all lenses are equally prone to lens flares. The higher the magnification of the lens, the more internal reflections of stray lights occur so zoom lenses are more prone to the lens flare effect. Wide-angle lenses are far less prone to lens flare.
How do you avoid lens flare?
If you use a lens hood it can reduce the amount of non-image light entering your shot.
Another option is to ensure the light source if behind your camera rather than in the direction in which you are shooting a photo.
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