An article about Col Mulquiney's PHOTOSTACKING can be found in the 2015-4 Summer Newsletter, pg 3. The following is an exerpt.
For me the idea of 'Focus stacking' is to create a single photograph which has a carefully designed depth of field, one which shows all the detail I want. Overall, this is done by taking many photographs (the 'stack') each with a shallow depth of field (but very sharp). Each has slightly increasing (or decreasing) focussing distances. If all photos are combined, only those parts of each overlapping photograph which are in focus, are combined into one photograph.