In the days before 9/11 I had the privilege of riding two approaches in 757s. The one that sticks with me was a day approach in lovely weather until we got to something like 500ft. We then descended into this lovely white cloud that got darker as we continued to descend. There was, essentially no visibility, nothing. Then just before touchdown some things on the ground began to form. Once the machinery had landed us there was visibility, but not much. Visibility is measured on the ground - it's the "Runway Visual Range". Visibility on the approach can be very different and the change during that final descent can be very sudden and it can get worse as well as better. My layman's view? I don't want to risk any interference with the autoland equipment so by all means ask me to turn off my phone etc. for 5 minutes.