Back focal length

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Focal length s' for a plano-convex lens between the vertex of the lens and the point of intersection of the green rays with the optical axis

The back focal length s or s' is understood in optics :

• In geometric optics, the intercept of a light beam in a meridional plane is the distance between the intersection of the beam and the optical axis from a reference point. As a reference point, one usually chooses the vertex (center point) of the surface where the ray was last broken (see also main plane ).
• The focal length of a lens is the distance between the image and the rearmost optical surface of the lens. The object is usually at an infinite distance. It is therefore the back focal length of the axially parallel incident light beam after it has passed through the lens. This focal length is only identical to the focal length for an infinitely thin lens in the area of paraxial optics . Otherwise the back focal length will deviate from the focal length due to the distance of the main plane from the rearmost optical surface and due to spherical aberration .