Guys I am having trouble with using GND filters effectively . I know the basic concept that you need to place the grey side on the brighter side of the scene and take a shot, what I am not sure is how do you calculate which GND to use the 3, 6 or the 9. I also know that a meter reading can be taken using spot metering in manual mode to work out how many stops difference there is my knowledge stops here. I am not sure what shutter speed do you set when taking an exposure reading in manual mode. I set the F stop to 9 and do not know what shutter speed to use to take the reading and after I have taken the reading from the sky and the ground how do I calculate which gnd to use....