Secret Camera Symbol

Most cameras have this strange symbol imprinted somewhere on the case. If you read the camera's manual, you know what it is but if you didn't, that circle with a line drawn through it marks exactly where the sensor of the camera is located.

It is called the 'film plane mark' and is helpful for people who take macro shots. Knowing exactly where the sensor plane (or film plane or focal plane) is inside the camera's body let's photographers know the exact distance between their subject and the film plane.