When direct light (sunlight, flash light studio light and anything else that can create flares) falls on the lens it creates white blobs in the photograph, popularly known among photographers as flares. Sometimes even when the camera is pointing to a direction that does not have a strong light source, a stray beam of light may reach the sensor. This can happen if a straight line is established between the front end of the lens and the light source. This light does not get refracted in the usual manner and gets reflected internally which then creates the flare on the image.
So what they do basically? They create less than normal contrasts; they can subdue the whole color saturation of the frame, seriously affecting the quality of the final images. At the same time they also make the picture look dull and flat. They can also make the subject look under exposed (where the reflected light is of larger intensity compared to the refracted light coming through the front elements of the glass.
Lens flares are however, not always a taboo, unlike popular believe, and some photographers have used them creatively (and intentionally) to recreate some moments through their lenses. However for the larger part they are kind of party poopers and needs to be eliminated when composing your shots.
Photographers have been using various techniques to reduce lens flares. Some of these techniques are quite simple and straight forward while others take a bit of adjustment. A lens hood immediately comes to the mind as a gear that is quite effective for reducing lens flares. Wide angle lenses have lens hoods that are designed to not obstruct the wide angle of view. As such they cannot afford to extend too much and cut down on the angle of view. Result is they are not as effective when it comes to reducing lens flares. Conversely tele lenses come with a protruding lens hood. They are quite effective as they can obstruct the intrusive sunlight more effectively. Landscape, wild life and architecture photographers using tele lenses rarely shoot without a lens hood on. However apart from lens hoods there are other methods of cutting down on lens flaresZooming
Zooming can sometimes correct the problem. This is of course is more helpful if you are using a compact superzoom camera or a DSLR with a tele-lens. At higher focal lengths the problem of flares is corrected to a large extent.
Using lens coating
On their part lens manufacturing companies have also done a considerable amount of work trying to counter lens flare while also making it possible to shoot directly into the sun. Magnesium fluoride and or silicon monoxide are two types of coating that are used on lenses to reduce reflections. In fact Canon uses a term called light wave interference phenomena to explain how they do it with the coating surface and the lens surface canceling out the light reflected by each one. Cheaper quality lenses are usually coated once or twice; however for correcting reflections from light of different wavelengths, more expensive lenses such as Canon’s L series use multiple coating. Today, it is possible to apply up to ten layers of coating on lenses.
Working around the problem
Some photographers even try to work around the problem and change their compositions slightly to counter lens flare and even make that an integral part of the frame. Artistic compositions such as a slight flare through the leaves of a tree or even a flare just sinning out of the corner of a building has been used in architecture and landscape photography very successfully.