I couldn't get Finder searches (Spotlight Saved Queries, aka Smart Folders) to search for photos with exposure times greater-than or less-than anything but integer seconds, which is pretty useless. Then just now I found that if I edited the Saved Query in a text editor (or Property List Editor), I could change the value for kMDItemExposureTimeSeconds to what I really want.
For example, I told Spotlight that I wanted to find exposure times greater than 0.03 seconds. In my text editor, the saved query read:
kMDItemExposureTimeSeconds > 0
I changed it to:
kMDItemExposureTimeSeconds > 0.03
I saved the file, double-clicked the saved query, and lo and behold, only those photos with exposure times greater than 0.03 seconds showed up. Also note that when I pushed the Edit button in the Saved Query, it showed Exposure time ... Greater than ... 0.03. But trying to set it (in this window) to 0.029 just snapped it back (in the Saved Query) to 0.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006020816350135