Focus Stacking / DFF
Focus stacking or deep focus fusion (DFF) is a method to increase depth of field (DOF) especially with macro subjects. Generally DOF is dependent on focal length, aperture and subject distance. With macro subjects the distance is usually rather short with a magnification of 1:1. Ths results in a rather shallow DOF. It can be increased by stopping down, i. e. using a larger f-number. But this is only sufficient for small subjects. Also, with f-numbers larger than 13 diffraction will occur which results in decreasing resolution.
Focus stacking is a way to circumvent those problems and enable really deep DOF. Therefore several dozens or more pictures need to be taken. The camera is mounted on a tripod with a macro sledge. This slege allows to move the camera steadily closer to the subject. So one image is taken every (for example) 1mm step. In the end all these images will be grouped into a stack and then merged so that only the in-focus parts remain. The result is an image with extremely deep DOF.