Try a scanner:
If it's really just about having an image of these seeds (for documenting them etc.) and getting the focal length right for all of them, then the easiest way to get really nice high-res images is with a scanner.
Leave the lid open and use in a dim room to get a nice pitch black background; close the lid if you want a white background OR put some paper on the seeds so as not to crush them.
I accidentally discovered this technique in high school (more than twenty years ago) -- you can do some really creative stuff with it. But I recently heard somebody using a scanner on the Frames photography podcast. Can't remember the artist's name of episode though.