The invention describes a focal length extender for a telescopic imaging system with interchangeable viewing eyepieces, including a cylindrical housing, which has a first and a second stop plane which are perpendicular to its cylinder axis in the direction of the light with associated connecting elements to the telescope body and for interchangeable viewing eyepieces and in which the housing has a relay lens arranged between the first stop plane and the second stop plane, wherein the relay lens is divided in two and consists of a first positive, neutral or negative lens element and a further negative lens element, wherein between the first and the further lens element, a beam splitter surface is arranged downstream at an angle to the optical axis of the imaging system and the first and further lens element together with the distance therebetween has an overall negative refractive power.