Making Use Of Lens Flare
Lens flare, where the sun/stray light strikes the front of the lens and creates blobs or streaks, can be a curse or a blessing. It can appear as an unwanted artefact, spoiling your wide-angle shots in particular, or it can be charmingly retro effect that you can manipulate to good effect in portraits (fashion photographers often take advantage of it to give a 'sun kissed' look).
How to avoid lens flare
Lens flare becomes much more of a risk when you are looking towards the sun with wide angle lenses, and you might not even notice it until after you have taken the shot. The easiest way of avoiding it (apart from repositioning the camera) is to fit a lens hood; these often come with a new lens, or you may have to buy them separately.
If you resent paying good money for an extra bit of plastic, you can use your hand or wallet to shield your lens from the sun, but make sure it doesn't get in the way. If you're a real masochist – or just good at Photoshop – you can also take two pictures, one with lens flare, another of the same scene with your finger blocking out the sun. Then blend the two exposures in Photoshop.
How to exploit lens flare
Personally, I don't mind lens flare too much and I'd rather exploit it than obsess about avoiding it. One of my favourite techniques is 'contre jour' (literally, 'against the light') where you put a tree or tall building in front of the sun. You can get some nice effects here.
Shoot at a narrower aperture, eg f/16, and the sun's rays look like a starburst – use a tripod and you can stop down even further while still keeping the shot sharp. Contre jour can also work well if you use it with a famous landmark such as Big Ben or the Eiffel tower. The building may end up underexposed/in silhouette but you can still make out its distinctive contours.
Lens flare can also be used creatively with portraits – skilled wedding photographers often take advantage of it. One technique is to place your subject so they are backlit by the sun, so light fills the frame.
Either move focus points around or lock focus and recompose to keep the subject's face sharp while experimenting with different angles of lens flare. Experiment with aperture width, too.
Opening the aperture wide open (f/2.8) gives you nice blobs and orbs of light, and as we mentioned, reducing the aperture, or stopping down, gives more of a starburst effect. Remember too that not all light is created equal, so lens flare during lovely warm evening or early morning sunlight will be more attractive than lens flare from harsh mid-day light.
It all depends on the creative effect you are after, however. As with so many aspects of photography, it's all about experimenting and moving your subject around (or moving around your subject) to make the most of the light.