For the measurement of the back focal length the distance between the rear focus and the surface is measured.
The specimen produces an image of the reticle in its rear focal plane, which is usually located behind the last lens surface. The measurement is done in two steps:
1) First the telescope with attachment achromat is moved such that the image of the collimator reticle appears on the camera.
2) In the second step, the telescope is moved such that the surface of the specimen is sharp in the view, The distance between the two positions is the back focal length to be measured: