I'll bet that straight line is actually "1" and the circle is a "0."
1 means on, or full, depending on the use
0 means off, or empty.
This is an educated guess..."on" and "off" are specific to the English language, so if the lens was made for use internationally (or even just in Europe), they may have gone to symbols rather than words so everyone can understand it.
Again, this is just a guess.