Abstract: A method of attaching a Gradient Index (GRIN) lens to an optical fibre in the course of making an optical instrument for use at a working distance. The method is especially suitable for making an optical instrument with an extended working distance. In the method, a test object, a GRIN lens, and an optical fibre are provided. The GRIN lens is placed at the working distance from the test object, and is axially aligned with the test object. The optical fibre is placed adjacent to, and axially aligned with, the GRIN lens to form an image of the test object at the proximal face of the optical fibre. The distal face of the optical fibre is placed adjacent to the proximal face of the GRIN lens. At least one of the GRIN lens and the optical fibre is moved axially with respect to the other until the image of the test object is in focus. This determines an optimum axial spacing between the GRIN lens and the optical fibre.