Back Focal Length

OpticStudio defines the back focal length as the distance along the Z axis from the last surface made of glass to the paraxial image surface for the object at infinite conjugates.

Note that if the image space is not in air (the Material column of the Image surface contains a Material name), the reported back focal length will be the distance from the Image surface to the paraxial image location for the object at infinite conjugates.

If no surfaces are made of glass, the back focal length is the distance from surface 1 to the paraxial image surface for the object at infinite conjugates.