A Victorian favourite, this pale emanation remains a standard in ghost images. These days, this ethereal-looking substance is likely to be flash light reflected by condensation on the lens or from the photographer's breath, says Wood. Savvy ghost hunters lurking in icy churchyards are careful to avoid this effect and ban smoking on the job for the same reason. Others may fail to notice a flash firing as the photo was taken. "People will swear the flash did not activate," says Wood. EXIF data stored in the camera's memory will reveal the truth.

Another recent phenomenon, the elongated silvery forms of vortex energy or "funnel ghosts" can also sometimes look banded like a tornado. It may be no coincidence that a textured camera strap hanging in front of the lens can create a similar effect. "Each photo should be judged on its merits," says Gordon Rutter, of the Edinburgh Fortean Society and author of Ghosts Caught on Film 3. "But the evidence is pretty convincing for camera straps being the culprits."

Camera lenses contain several glass elements. When pointed towards a bright light they can reflect or scatter it onto the camera's sensor. The result is a faint glow or washed-out region in an image, or a single spot (or line of spots). If a flare spirit disappears when the lens is shaded or is hexagonal in shape (like the lens aperture), it probably has optical rather than unearthly origins.

Odd balls of light known as orbs began to appear widely in photos in the mid-1990s. To believers, they are a sign of spirit energy. Sceptics disagree.

The lens systems on many compact digital cameras are not designed to focus on a subject closer than a few centimetres away. Dave Wood, from the Association for the Scientific Study of Anomalous Phenomena, has shown how insects or dust appear out of focus when illuminated by the camera's flash, creating bright spheres.

Flash photographs taken through glass can create phantom reflections, an effect known as Pepper's Ghost. But camera flashes can have other effects, sometimes reflecting from objects outside the frame to produce diffuse patches of light or making distant objects - especially if they are flat or reflective - stand out in unexpected places. Similarly, slow shutter speeds can give moving figures an ethereal appearance.

The human brain is optimised to recognise faces, which could also explain why we are so good at picking out meaningful shapes in random patterns. This phenomenon, pareidolia, could be responsible for a host of otherwise unexplained sightings, such as the face of the Virgin Mary on a toasted cheese sandwich. In the same way, a chance arrangement of clothing, shadows or rocks could account for ghostly figures or faces in photographs.