That is a very interesting question which raises another one. Why do you think we see ghosts with a digital camera? I have not seen one thing aside from maybe some TV shows that have shown that ghosts exist much less can be observed with a camera.
What a camera CAN see is infrared, which we can't. Flash a remote at a digital camera and you will see a red light. Flash it facing your eyes normally, no light. OOOOOoooo scarrry ghossstt magic light must be the answer! This is pretty much how all the myths start, someone sees something they can't explain and it's immediately ghost this and gods that and vampires, etc...
A similar thing would be seeing the pot of gold at the end of the rainbow. Never seen one with my eyes, but it's all over the place in books and cartoons.