Sweeping Lens Tip: If you're buying lots of Stealth Wards but not killing any with Sweeper...

...just change your trinket to the Warding Totem. You'll save a lot of gold that way.

This of course may be different on a case-by-case basis, but in general, I sometimes see someone had bought a whole lot of wards for vision so they could use the Sweeping Lens, but they never actually sweep any or use it.

Don't forget that upgrading your Warding Totem or Sweeping Lens at level 9+ makes it far more efficient.