Why can’t the Canon 100 micro lens focus after adding an extension tube?

The function of the extension tube is to increase the distance from the lens to the imaging screen. At this time, the ratio has exceeded 1:1 and can reach a 2:1 relationship, that is, an object 1CM long can be placed on the imaging screen To obtain a 2CM image, according to 1/U+1/V=1/F, the object distance will be greatly reduced, and the focal plane that can be imaged is only a very short section. Therefore, at this time, the camera must be fixed with a tripod and carefully pass the focusing screen Check the focus, or use the Canon camera's display screen plus the magnifying glass function to achieve focus, otherwise you will not be able to find the focus. In addition, the aperture needs to be narrowed to obtain a certain depth of field and avoid out-of-focus situations.