all the sifu here pls help !! .. when i m using a extantion tube for my d 90 , 18 - 105 lens ... for macro shoot seems like having focusing problem ... try 10 times only 1 time can focus ... change my lens to tamron 17-50 .. still the same .... but i don t know what s the name of the extantion tube ( i bborrow from my friend )...