Anything floating in the air, be it dust, moisture, rain, or insects, will reflect the light of your incredibly-bright camera flash. If it happens to be at a distance from your lens where it's out of focus, it'll manifest itself as a larger "thing that wasn't there" when you took the picture. Not grandpa.
Let's create some ghosts.
Ingredients:
- A Pinch of Powdered Sugar (our dust)
- Camera with Flash
- A few minutes of your time, to waste doing unimaginably unimportant things like taking fake ghost photos.
Here's a couple of before & afters taken in good o'l ghost-hunting fashion, using an iPhone 4 to capture tiny motes of sugar in the air.
The clarity, shape and color of your orbs will all vary depending on a number of things, including any movement (flying insects create "ghost trails"), your lens type, your aperture setting, and even the number of aperture blades inside your lens. Basically, it all works on the same principles that create really great bokeh (like in the photo below), with one difference: it just happens accidentally.
So, next time you're watching some crappy ghost show that uses orbs as photographic proof of the afterlife... just change the channel.