Focusing an image in a DSLR viewfinder using a ground-glass screen may not be sufficiently precise for micrography work. However, there is a particular microscope port-to-sensor distance which provides a parfocal sensor image with a microscope's binocular eyepieces. A parfocal lens is one that remains in focus as the magnification is changed. Consequently, with an extension tube of the correct length inserted between the camera adaptor and the microscope, there is no need to vary the focus setting of a camera's lens. If the correct length of tube is not available from the manufacturer of the microscope, it may be necessary to have one custom made.