Good point... I think the short answer is, all of the above.
Although, I see a lot of it as genuine mistakes and misreads - especially when you hear phrases like, "hey that wasn't in the photo". This really points out to some kind of natural
artifact that often occurs with the medium.
It's also quite interesting to note that the bigger the
flash the more ghostly effects you'll find. I remember hearing about that on the SGU podcast. They said that this was up on one of the major paranormal web sites. In fact the paranormalists recommended the ghost hunters to use an even
bigger flash. In order to ger more ghosts.
However, it was being thoroughly laughed at by skeptics - so they took the phrase off their website. I can't remember who they were now - I'd have to listen to the podcast again. But I found it hilarious.
Technically the camera lens mimics the human eye - they are not designed to pick things up that we can't see. So it just goes to show when these things
suddenly show up it's usually due to a flash effect, or human contamination, etc.
Cheers,
R.
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"Extraordinary claims require extraordinary proof." - Marcello Truzzi