What is a lens flare?
Lens flare is a bright light that causes undesired artifacts in your picture, which is usually a polygonal shape like starbursts, circles, or rings in a row across the image. It can also be in the form of a glare across the whole picture, which reduces contrast. The shape of the lens flare is dependent on the lens diaphragm. Typically, lens flares are caused by the sun or bright overhead lights such as spotlights.
Lenses contain several glass elements to ensure good quality pictures, and the actual flare is created when light reflects and scatters across various lens elements before it reaches the digital sensor or film.
A good lens hood can eliminate flare caused by stray light from outside the angle of view; however, most lens hoods do not extend far enough to block all stray light. Zoom lenses can only be large enough to block stray light at the widest focal length.
An adjustable bellows can be used, which is a lens hood that matches the field of view for a given focal length. You can also use a piece of paper or your hand to block the light, but this can be problematic at best.
Fixed focal length lenses are less likely to have lens flares than zoom lenses because zoom lenses have more internal surfaces for light to reflect on. Wide angle lenses are designed to have fewer lens flares because the sun is likely going to be in the picture more often. Also, newer lenses have more anti-reflective coatings to reduce lens flare even further.
Contrast problems resulting from lens flares can be corrected in Photoshop or other good graphics software by adjusting the levels or curves of an image. If a flare is desired, but not present, it can also be added in post production by selecting the lens flare filter in Photoshop. There are several simulated focal lengths which can be used to produce starbursts and other artifacts.
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