As my online petition to Canon, Nikon and other manufacturers of interchangeable lens cameras to rename the inappropriately monikered "Focal Plane Mark" gains steam (as many as two people have now signed), I figured it was time I went ahead and blogged the issue.
That little circle with a line through it on top of the camera body next to the pentaprism, which indicates the location of the film or image sensor, according to Canon and Nikon, is called the "Focal Plane Mark".
It is not sufficient to call this misleading. It is blatantly incorrect. As I'm sure you're all aware, the focal plane, or more accurately the rear focal plane, is a theoretical flat surface positioned relative to the rear nodal point of a lens. More specifically, the plane represents the surface upon which rays of light reflected from a point of infinite distance and entering the lens converge back to a single point. (The distance between the focal plane and the rear nodal point of the lens is equal to the lens's focal length. ) Rays of light reflected from points of less than infinite distance converge to a single point behind the focal plane.
As such, the position of the focal plane coincides with the film or image sensor only when the lens is focussed to infinite – as the focal distance decreases into the finite, the position of the focal plane correspondingly moves forwards.
So "focal plane" clearly does not describe the static location of the film or image sensor in anything but the most tenuous way, making the term "focal plane mark" a fallacy. Luckily however, there does exist a term that describes the position exactly – it's called the "image plane". It therefore makes sense that the mark indicating its location also be renamed the "Image Plane Mark", as that's precisely what it is.
So next time you're thinking of purchasing an interchangeable lens camera, why not badger the staff who have absolutely no authority on the issue over why it should be renamed. If we make enough noise, maybe one day the fat cats upstairs will hear us and get onto doing something about it.