For most standard lenses the working distance (WD) is not a fixed parameter. The focusing distance can be changed by adjusting a specific knob. Nevertheless, there is always a minimum object distance (MOD) below which focusing becomes impossible. Adding an extension tube (see Fig. 46) between the lens and the camera increases the back focal length, making it possible to reduce the MOD.
This also increases the magnification of the lens or, in other words, reduces the FOV. While very common in the vision industry, this procedure should be avoided as much as possible, because it degrades the lens performance (resolution, distortion, aberrations, brightness, etc.). In these cases, it is recommended to use lenses natively designed to work at short working distances (macro lenses).