rays that enter the system parallel to the optical axis are focused such that they pass through the rear focal point Source Wikipedia
If the medium surrounding the optical system has a refractive index of 1 (e.g., air or vacuum), then the distance from the principal planes to their corresponding focal points is just the focal length of the system. Source Wikipedia
The front and rear (or back) focal planes are defined as the planes, perpendicular to the optic axis, which pass through the front and rear focal points. Source Wikipedia
