Photographing ghosts
Having spent close to twenty years in the photography business (as both a photo lab/darkroom specialist and a photographer), I have seen first-hand how many photographic errors, tricks, and camera malfunctions are possible.
Oh, if I had a nickel for every person who claimed they had a ghost picture...
A few examples:
- A blurry, unidentified ghostly object in the shot
Camera strap, thumb, or the photographer's own long hair passing in front of the lens. NOT A GHOST.
- Wispy spirits swirling around in the photo
Somebody is smoking a cigarette off to the side... or it's cold out and the photographer's own warm breath is visible. NOT A GHOST.
- "Ghost orbs"!
Moisture in the air is captured and illuminated by the camera flash. Our eyes may not see the moisture at the time, but the flash will pick it up. NOT A GHOST.
- Mysterious lights/reflections
Mirrors, windows, reflective stripes on backpacks and sneakers... even drinking glasses and dinner plates... can all cause odd reflections in photos. Reflective items don't even necessarily need to be IN the shot to be seen- a sort of ricochet effect can happen. NOT A GHOST.
- Blurry, partially transparent objects/figures
Caused by a slow shutter speed, whether used intentionally, or selected by the camera to compensate for lighting conditions, or caused by a dying battery or other malfunction. Camera shake, blurriness, and "trails" are fairly inevitable at shutter speeds of 1/60th of a second or slower. NOT A GHOST.
I would personally estimate that 95% of the "ghost pictures" I've seen have a reasonable technical explanation; the other 5% I can honestly say could not be explained scientifically. (It's in that 5% where things get very interesting.)
Overall, critical thinking is necessary in evaluating alleged ghost pictures. Understanding the technical aspects of photography helps, too. That said, I am open to the possibility that ghosts may in fact exist, but am not convinced they can necessarily be photographed.