I read this at another site and thought it was quite interesting

One of the leading theories concerning what orbs are is that they are not the spirit at all, the orb is the energy being transferred from a source to the spirit, so they can manifest. It may not even be a conscious thing by the spirit, just the natural way they get energy.
According to Laws of Physics energy being transferred would assume a natural shape of a sphere.
It went on to tell of an experiment with 10 ppl using 10 different cameras, film and speed etc on a cemetary walk with a psychic and the findings in the developed film. All get the film developed at different places, say only half get orb photos, if it was water or dirt on the lens then 5 ppl did not clean their lens and had spots in exactly the same places on their lens. As they are developed seperately then developing process can be eliminated and digitals don't have film processing errors. When an error occurs in a digital image it is more likely to be square and not semi-transparent.
If it was dirt and dust being stirred up then why in a sequence of photos do you get some normal photos with NO "dust or dirt" in them all photographers present should get the same pictures.

Thats what I rekon too! And no person would actually take "ghost" photos in the rain or mist!