You know, I've oftened wondered the same thing Wilhelmina. In my opinion, I think that most orb captures can be accounted to something natural, per say, like dust or pollen. It's often hard to tell but usually if your capturing a dust orb there will be a lot of "orbs" in the picture. Something that would be more legit would be a singular orb or something with some substance to it. I'm sure you'll get some better answers from some of the others on the board. I am by no means a paranormal expert. Not yet anyway
I'm not sure why we can capture them on film but can not see them with the naked eye. I know that I've started getting a lot more "orbs" when I started using my digital camera.
I have to agree with Hallow, it sounds like there might be something around you that is draining your camera batteries. I just find it to be quite a coincidence that you've had that many brand new cameras that just stopped working, although I suppose it could happen. Maybe just a streak of bad luck with cameras?
At any rate, I'm excited to see what some of the other Spiritkeepers have to say on the subject. =)