In 2000, Noel T. Goldsmith wrote "Deep Focus; a digital image processing technique to produce improved focal depth in light microscopy" (Image Anal Stereol 2000; v19; pp163-167 and this PDF). The article outlines and demonstrates how software can detect the most in-focus parts of multiple photographs of the same subject and stack them together to produce one image that is in focus throughout. It is the 'through-focus' analog of panoroma stitching.
It only took a year or two for the first commercial products to appear, and by 2003 there were several. One that seems popular is Helicon Focus. The three images that follow are from a review of Helicon Focus at Digital Photo Pro. They suffice to show the principles.
This is the first of ten images in a "stack" taken over the wide focal range of this sunflower field.
This image, the tenth in the stack, is close to what you'd get by focusing near the far end of the field, and letting the closest flowers go out of focus.
With the software, the whole field appears in focus. The original paper by Goldsmith was applied to microscopy, and that is still its most useful arena, for depth-of-field problems are the greatest there.
This image, from Concept 2 Innovation, could not be taken any other way. It is actually a stitched mosaic (panorama) of several through-focus stacks processed by Helicon Focus. Of course, this is quite a bit smaller than the research photograph, which shows great detail over and throughout the entire fly. It think of such a photo as a kind of gigapixel image with three dimensions. Producing two such images at an angular difference of 10° will yield a stereo pair of fully focused images. Heaven for a microscopist!
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