Several of the photographic formulæ in f/Calc involve the distance from the camera to the subject. The camera, however, is not flat, so what point on the camera am I talking about? The film plane? The front of the lens? Somewhere in between?
Confession time: I don't really know.
If you go and talk to an optical physicist about lenses and subject distance, they will talk about things like the optical center of the lens and the lens' front and rear nodal points. Even if you understand these concepts, it's still difficult to find out exactly where they are on your camera. They are not calculatable without knowing the precise optical properties of your lens. With zoom lenses, these points can actually shift forward and backward along the axis of the lens!
This distinction usually only matters in macro photography, since the subject distance is so small. Even then, though, you can usually compensate for not knowing these bits of trivia simply by carefully examining the results in the viewfinder before tripping the shutter.