Plastic calipers

Bel-Art The Scienceware® Company - VERNIER CALIPERS

This is the only source for all-plastic vernier 0.1 mm calipers that I can find these days. Forestry Suppliers and all the other usual suppliers have moved to cheap plastic dial calipers (dial fills with water and stops working) or to digital calipers (also not so good in water).

If you're working in an area where you might only occasionally get wet, you might just drop the $16 on Harbor Freight's house brand calipers, which read out to 0.01 mm, and consider them "disposable".
Harbor Freight digital calipers

Macro image stacking

http://www.hadleyweb.pwp.blueyonder.co.uk/
This site houses a little program called CombineZM, which incorporates routines for combining several macro images into one image with a huge depth-of-focus.

To use it, you simply take several pictures of your subject, and between pictures move the camera slightly forward to put a different slice of the subject in sharp focus. Alternatively, you can keep the camera still and slowly adjust the focus to put different slices of the subject in focus. The resulting group of images is then fed into CombineZM, where the program automagically spits out a new image that has everything in focus.

For example, this shell (~1.25" across) was shot in 9 frames, and I moved the camera slightly forward between images. (50mm, f/8, 1:1 magnification):

Only the front edge of the shell is in focus in the picture, as that is all that f/8 at this magnification will give me.

The combined final image, made of up 9 separate pictures, looks like so:

Now the whole shell is in focus. A little but of clean up around the edges of the shell was necessary in Photoshop, but the final image gives a much clearer picture of the entire shell. This works on the original files, so the final file is ~10 megapixels, suitable for making large prints.