Assuming that the lens is surrounded by air or vacuum (refractive index 1.0), the focal length is the distance from the secondary principal point (which in this case is also the secondary nodal point) to the rear focal point of a lens.
from the front focal point of the system to the vertex of the first optical surface.[1]
Back focal length (BFL) or Back focal distance (BFD) is the distance from the vertex of the last optical surface of the system to the rear focal point.
 See also: Sensor, Photograph, Nodal point, Lenses, Telephoto
  
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