Even "lighter" is the magnifying lens incorporated into the platform of many compasses.

Along the lines of starting fires with lenses, there are stories, perhaps urban legends (or maybe rural legends) of brush fires being started by suns rays inadvertently focused on super dry brush here in SoCal by broken bottles, etc. I have seen this mentioned in print but I don't know of any proven cases. Probably don't want to magnify this issue out of proportion.