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back focal length

= BFL)
distance on the optical axis between last active optical surface and the sensor when the object is at infinity.

The value is only valid in paraxial optics, ie for objects close to the optical axis.
Further off the optical axis, the focal distance of distant objects is affected by the spherical aberration.
(back focal distance = Back Focal Length = BFL)

Note:
Not to be confused with the effective focal length EFL!