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What is ghosting and what causes it?
Answer provided by Alan Jones.
We have all observed undesired, multi-sided spots of light in images when shooting silhouettes and other pictures with strong backlighting. The problem can also occur when shooting toward the sun early or late in the day. These so called “ghost images” are symptomatic of lens flare. Light streaks in an image are also symptomatic of lens flare.
What causes it?
Lens are designed to focus the incoming light on the camera’s sensor. Sometimes light entering a lens is reflected internally at glass-air boundaries rather than continuing on its intended path of travel. The deflected light bounces around inside the lens and on reaching the camera’s digital sensor causes light streaks and ghosts of the diaphragm imposed on the image. Basically, lens flare is caused by too much light flooding through the camera lens.
Is lens flare and ghosting affected by lens type?
Most of us use compound lenses made up of several lens elements rather than a single element. Because there are more internal surfaces from which light can reflect, the potential for flare increases as the number of elements in a lens increases. Zoom lenses, for example, often contain more lens elements than fixed focal-length lenses. Therefore zoom lenses are generally more susceptible to lens flare than fixed lenses.
Wide angle lenses, because of their greater angle of view, are also more susceptible to lens flare than lenses with a narrower angle of view.
There is also the potential for increasing lens flare when we attach close-up lenses; or lens filters, such as UV, polarizing, and neutral density filters; or other types of lenses, such as teleconverters, to the camera lens.
How do lens manufactures reduce lens flare and ghosting problems with their lenses?
To reduce flare and ghosts, manufacturers coat lens surfaces with a multiple-layer anti-reflective coating. When lenses scatter less light, image quality is improved because they are sharper and have better contrast.
How can photographers reduce lens flare?
By using a lens shade, changing the camera angle when shooting toward strong light sources, and by shooting from a shaded area or in the shadow of an object. Shadows can be produced by positioning a hand, a piece of paper, a hat, etc. above or to the side of the lens to block or reduce the amount of problematic light entering the lens. Other ways to reduce lens flare are to keep lenses and internal parts of cameras clean and, when shooting into the sun, keeping the sun near to the center of the frame.
References:
Lens Flare: What is it and how to reduce it