This post was triggered by a couple of recent claims that something paranormal -- probably a ghost! -- was photographed.
The first case concerns a photograph taken by "paranormal researcher" David Bennett. It shows a woman sitting on a gazebo, only we are told that it could not just be a woman on the gazebo, because "there were four of us present and no one saw anyone there with our
physical eyes. If she had been an actual physical presence, someone
would have noticed her."
In the second case, a woman took a photo of her pool. Near the center of the photograph is a small pool house, in the window of which is what might be a face peeping out. "She assures us there was no one in there. 'I was all by myself sitting
by the pool. My husband wasn't home and my son was across the street
with his friend.'"
In the first case, it is obviously suspicious that the person who took the photograph for no apparent reason is a self-described "paranormal researcher"; maybe this is simply a lie. In the second case, what looks like a face is very blurry, poorly lit, and seems to be partially obscured; it could easily be a case of pareidolia. However, if we take the two photographers at their words, what they're saying boils down to, "It is more likely that this is a ghost than that there was a flesh-and-blood human here whom I did not notice."
How reasonable is that assertion? Probably not as reasonable as one might think. It turns out that we perceive a good deal less than we think we do. In much the same way that the mind "covers over" the blind spots in our vision with "good guesses" based on the surrounding image, so that we don't notice the blind spots, it uses good guesses about what's going on to decide what is important for our attention and what is not.
Take a look at The Invisible Gorilla for some surprising demonstrations. (I think some of these are a little unfair; after all, we have been trained by movies and TV to "see" what they mean, not necessarily what is shown, but the demonstrations are still quite striking.)
On a more serious topic than alleged paranormal photographs, this research has some disturbing implications for the reliability of eyewitness testimony and the need to be extra careful when supervising children.