Lens flare occurs when the direct source of light in the photo his the front element of your lens. It is characterized as the circular blobs or streaks of light across your frame. Lenses with more elements, such as zoom, tend to have greater lens flare.
Lens flare can be related to the unwanted glare in a human eye. When viewing very bright lights or highly reflective surfaces, you have a glare in your eye that obstructs your vision.
In addition to blobs and streaks, the bright light can also wash out the image. If the light source is not in the field of view, it creates a haze that reduces the overall contrast of the photo.
How to avoid flare
-Buy a lens hood. Lens hoods work by casting a shadow over the front element of the lens. Hoods also help protect your lens from accidental knocks and scratches.
-If the camera is on a tripod, you can shade the front element with your hand or card during exposure.
-Take photographs in the shade. If the subject and camera are both in the shade, you will not get flare.
-Keep the lens clean. If your lens is dusty, dirty or greasy it will more likely create flare. Your vision is clearer if the lens is clean.
Gibson, Andrew. “How to Prevent Lens Flare – Digital Photography School.” Digital Photography School RSS. Digital Photography School, Web. 1 May 2014. <http://digital-photography-school.com/how-to-prevent-lens-flare/>.