Interesting. Flickr doesn't do stemming so
synths is not the same as
synth. Each term is discrete - it's up to the image poster to use multiple tags for the same picture. Read the following post below if you don't know what the heck I'm talking about. : )
Update:
I sub to the Flickr blog for updates on the site's features. They just introduced something called clustering which actually might help the above. Funny how that works sometimes...
So... if you append /clusters to what you are searching for you will get the other categories that come up for the pictures in the set. Try http://flickr.com/photos/tags/synth/clusters for an example. You will see different clusters for music, keyboard, korg, synthesizer and moog to start. Pretty cool. And no I have no affiliation to Flickr whatsoever.