Board Thread:Questions and Answers/@comment-5836726-20131016212310/@comment-24220404-20131127071358

If you remember when Lelouch first got his Geass he wanted to study it, he said that he needs direct eye contact but transparent objects like glasses present no problem (it can also be reflected as we see in his confrontation with Charles), it has a range of 270m and the victim has no memory of events while under the influence due to damage to the brain.

http://youtu.be/Sn2axCKNIFs?t=15m28s

When CC give Lelouch his contacts she says it is a special lense that blocks Geass however if the Geass grows stronger it may not be enough so strong optical Geass' like Charles' may still work