Although the macro lens adapter and microscope mount work well enough, the relatively small sensor and lens in my Canon SX230HS make for a razor-thin depth of field:
Those are, of course, millimeter divisions on the ruler.
A bit of rummaging leads to the notion of Focus Stacking, which involves taking a sequence of images with identical exposure settings and different focus points, then compositing the in-focus parts of each image to produce a single image with everything in focus. Although some examples show a manual process involving layers in, say, The GIMP or Photoshop, I think an automated process would be better.
Given that I have a Canon SX230HS camera, the first step is to download the proper version of the Canon Hack Development Kit, unpack it onto a spare SD card, and get used to it.
As it turns out, the focus bracketing works exactly as intended, but doesn’t do quite what I need: it changes the focus in linear steps by adding a constant bracketing distance. The macro lens adapter drags the “infinity” focus point inward to maybe 15 mm beyond the innermost focus point, but the camera’s focus range still shows 1 m to ∞. Stepping in 1 m increments generates a bazillion pictures that don’t differ by much at all after 5 m, but you still need a few near the far end.
However, it seems the only way to get a bazillion pictures is by holding the shutter button down with the drive mode set to Continuous, as the camera’s Custom Timer mode has a 10 shot upper limit. If I must do that, I may as well adjust the focus manually: the assumption being that the camera shall be firmly mounted to keep the pix in alignment, which currently isn’t true in any of my setups and certainly won’t be true with my finger on the button.
The camera already has exposure bracketing, although not to the extreme range available through CHDK. RAW images (or the roughly equivalent DNG format) might come in handy at some point, but right now they’re just a temping digression available only through CHDK.
If I’m going to keep using CHDK, I must conjure up an artificial NB-5L battery with an external power source. Those cheap eBay batteries work fine for the usual duty cycle, but constant zooming & focusing & suchlike chew them pretty hard…