But these days, with current camera design, lens flare is taking over from orbs as the number one cause of false positives in photographs purporting to show paranormal activity. The evidence for this claim is the proportion of photos sent to us for examination for which lens flare is the most obvious explanation for the phenomena photographed.
Lens flare is caused by (1) any light entering the camera lens that does not form part of the image being photographed (non-image-forming light), and/or (2) bright light sources - natural or man-made - within the area photographed. The stray light bounces around inside the lens, internally reflecting from its optical surfaces and often acquiring colour from the microscopic anti-flare lens coatings. Sometimes the shap of the aperture blades within the lens have an effect.
Lens flare is much more various in appearance than orbs and so is harder to exactly identify. It can also cause orbs by either striking dust or other marks on or within the lens, or by creating optical artefacts. There are two such artefacts in the photo below, which was recently sent in to us for analysis.
Visual pareidolia - more info:
Other pareidolia photos on Strange Occurrences site
More about orbs: photo examples on Strange occurrences site
Orbs Explained - orbs as false positives, article by James Gilberd
Lens flare, if not actually producing artefacts in your photo, will generally deteriorate the photo's quality by slightly fogging the image, reducing contrast, colour saturation and definition, so you want to minimise flare for the best quality photos in any case (paranormal or not).
As a professional photographer engaged in trying to produce the clearest possible photos (usually, anyway), lens flare is something to always be aware of. With a DSLR or SLR film camera, the lenses usually come with a lens hood, and this should be used for every photo taken (not stored reversed over the lens!). In situations where the lens hood is unavailable or inadequate, use your hand or (preferably) some dark, flat object to shade the lens from the sun or other direct light source. Hint: if you can see the light hitting the front surface of your camera lens, you're going to get lens flare in your photo.
With compact cameras (most have no lens hood provision) or phone cameras, using your hand to block out the source of the flare is a good technique. Move your hand closer in until you can just see it in the picture onscreen, then withdraw it slightly. (Try taking two photos, one with and one without using this technique. You will see a marked difference in quality.) If the light souce is actually directly in your photo, this will not help: you are going to get lens flare.