It's an old SLR camera trick to make a 1:1 macro lens by adding a second lens of the same focal length (for example, 50 mm) arranged front-to-front on the existing lens. You can get a "reverse mount adaptor ring" that screws into the filter threads on both lenses, for this purpose.
You can do the same thing with the R-Pi camera. You start with two R-Pi camera modules, unscrew the lens from one of them, and place it on the second camera so the lenses are front-to-front. Now you have a 1:1 macro lens and the pixels in your output image will have the same spacing as the sensor pixels, that is 1.4 microns. Which is a pretty high resolution for simple mobile phone camera! The focus plane is only about 1 mm above the lens surface so you probably have to use lighting from the back, rather than the front. Below example shows a random hair. By measuring the pixels on the 1:1 crop you could measure its diameter pretty accurately (assuming both lenses really do have the same focal length).
https://picasaweb.google.com/lh/photo/s ... directlink
arrangement of lens on top of standard Pi-camera
https://picasaweb.google.com/lh/photo/2 ... directlink
full frame field of view. (light fuzzy blob in background is out-of-focus desk lamp directly above)
1:1 crop of previous image, showing some detail on surface of hair shaft
The depth of focus is super-tiny, needless to say. But with the camera being connected to a computer, a focus-stacking algorithm seems like a natural accessory (summing together a range of images taking the best-focused parts of each). That does probably mean a stage mount allowing you to move through focus in a controlled way, maybe even with a stepper motor.
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