Lens flare is often thought of as shafts of light that dart across a scene when a pinpoint source is shined directly into the lens. The problem is that not all light sources are pinpoint, and so not all flares are this noticeable, dramatic, or even desirable.
And yes I know, sometimes lens flare is desirable. So, if what you're after is a dramatic impact that only a shaft of flare can provide, go ahead and use it. Most of the time, though, photographers want to keep lens flare out of their scenes-and some of the time they aren't even aware they're getting it.
Particularly when working with diffused light sources-like a cloudy sky or a softened strobe-lens flare can insidiously manifest itself as a general loss of contrast, saturation and detail in an image. These aren't typically things that photographers want to lose from their pictures, so they work hard to fight flare. There are several things that can be done in almost any situation to minimize destructive lens flare-whether it's the glamorous shaft-of-light variety, or the awful insidious kind.
1. Hoods & Shades
The reason so many SLR photographers equip their lenses with hoods or shades is simply to fight lens flare. Fixed focal length lenses are often paired with equally uniform-looking lens hoods, and zooms sometimes sport tulip-shaped hoods that allow the shade to work at a variety of focal lengths. Obviously the fixed focal length hoods provide more solid shading coverage, but at the expense of working with a less versatile lens. The first step to fighting flare is to make sure that all your lenses are protected with shades-even if they're the inexpensive rubber flip-out kind. (When working with a tulip shade, make sure it's centered correctly. An off-axis hood can vignette the edges of the frame.)
Even with a lens hood in place, sometimes the best shooting angle still allows some of the light to shine directly into the lens. By the way, a great way to check for flare is with the camera on a tripod to look at the lens. If you can see the source reflected on the front element, you're getting destructive flare. That's why studio photographers and Hollywood movie productions, and just about everybody else with a camera and a budget rely on flags to block stray light from entering the camera. Even if you've done a good job of keeping the source itself from shining directly into the lens, sometimes powerful reflections off your subject or surroundings can create equally damaging flare. Flags come in handy in these situations too, when no lens shade could cover every angle.
So you're not a Hollywood producer or even a big budget photographer? No worries. Flags can come in a variety of shapes and sizes-from fancy framed black-cloth versions to cut pieces of cardboard, or even just a folded piece of paper. But the ultimate in-a-pinch flag is with you all the time-your hand. It's easiest with the camera on a tripod, but certainly still possible if you're handholding. With the sun at just the right angle, placing your hand above and just out of frame is a great way to keep the light from your lens. Looking through the viewfinder you can easily see if you're in the frame or not, and looking at the shadow of your camera on the ground can easily show you if you're casting your own hand-made shadow directly over it.
4. Hide the sun
If the sun is actually so close to the frame as to make it almost impossible to flag, consider using elements in the scene to block the sun for you. Subtle movements can put the sun behind a tree or other structure without dramatically altering your chosen composition. If the sun is in the frame, it's especially important to hide it behind anything in the scene, otherwise you will definitely create flare. But even with the sun out of the way, it can still clearly do damage. Set up your camera in a bit of shade created by a tree or building-especially since you're then not compromising your composition.
5. Change your composition
If all else fails, you've got a decision to make: do I want to keep this composition and live with the flare, or change my composition to eliminate it? Sometimes zooming in or just moving a few feet one way or another is enough to get rid of that nasty flare. But then you're compromising your composition. That's why it's a decision only the photographer can make, and why it should be saved for next to last.
6. Change your angle-moving until the light is behind you.
The last best option is to completely change the camera angle, moving until the light is no longer in a position to cause flare. Obviously though, you're compromising your original compositional vision. But if you can't have flare, and you've tried everything else, just move the camera and change your shooting angle until the sun is closer to shining on your back, rather than right into your lens.