How to keep your scenes clear of errant light
Lens flare happens when non-image-forming light enters a lens and falls on thefilm or sensor. If the light is well diffused it lowers the contrast of the image.
If the light isn't diffused, it can appear as bright spots, arcs, or other shapes. Flare is most problematic when there is a source of bright light (e.g. the sun) just outside theframe.
If your lens doesn't have a lens hood attached, light anywhere in front of the lens can getinto the lens, bounce around, and produce flare. A lens hood shades the lens from most of this light, allowing only the image forming light to enter the lens.
Unflare advantage: With the sun just outside the frame, the flare in the image on the left is obvious. Shading the front of the lens from direct sunlight with a hood greatly reduces flare, right.
Lens hoods don't always provide optimum shading of a lens. This especiallyapplies to zoom lenses used at longer focal length settings. When using a fully-extended zoom, don't be afraidto add extra shading with your hand. The effect is often visible through the viewfinder and as long as your hand doesn't show in the frame, you're safe!