I've only just bought these and when I first tried them on I thought they were OK. But after playing for 10 mins on court I soon found them to be unbearable and unwearable.
They won't fall off because the arms are rubberised and curved, bending inwards grasping your head, I quickly found this very uncomfortable, almost pinching behind the ears. I also found the lens' to be slightly magnifying making me a little nauseous.
BTW - the reason the lens' are black in the photo is because the original photo has a black background and this image has obviously been "cut out" from that. They are, as other have pointed out, just transparent.