Found this, which seems to go some way toward explaining it - http://www.fsona.com/product.php?sec=safety http://www.fsona.com/product.php?sec=safety Presumabl

Found this, which seems to go some way toward explaining it - http://www.fsona.com/product.php?sec=safety http://www.fsona.com/product.php?sec=safety Presumably when you get down to 1550 nm, the transmission of light through the lens is a lot lower.