From what I understand, you have part of it right, ernicus. Yes, it's an issue of 1:1 ratio. But that doesn't mean that it fills the entire sensor. I think of it this way: A 1:1 ratio is achieved when the size of the subject in the resulting photo is proportionally the same as the subject compared to the sensor. Say you have an ant that is 1/4 the size physically as the sensor. Then the resulting 1:1 photo will have the ant taking up 1/4 of the photo. Does that make sense? I could be wrong, though...