At our shop, we use the full face shields also (for oxy/fuel cutting and brazing), arc welding requires a darker lens as the other replies have indicated already. They come in #3 and #5 shades.
John Wright
Another thought...My welding shield is one of those automatic darkening types and it defaults to a light shade perfect for cutting/brazing and darkens to a dark shade when you strike an arc with an arc welder.