To gohjialong :
Thank you for your C&C ;p
I try to explain what I know about Neutral Density Graduated Filter (or some call it ND Gradient Filter or ND Gradual Filter), please pardon me if there is something not so ideal about this short explanation.
ND Graduated 4 Filter (gray color) is just one among the a lot of number of ND Graduated filters those are available in the market, usually it is used to block light partially at some certain percentage (%) number so it allows us to get the photos with changing the balance of the brightness of scene or object that we shot.
Here is the link of one band / company that produce the filter, so that show the image of what the ND Graduated looks like (they call it ND Gradual Filter) :
http://www.cokin.com/ico3-p1-6.html
Here is the link to show how to put / attach the filter to the camera ( in front of the lens) :
http://www.cokin.com/ico15-A.html
It very effective when for outdoor shooting we want to reduce the brightness of the sky (which is to bright) vs to increase the brightness of the ground object like a building, lake, tress, etc( which is a bit dark), so we put the ND Graduate filter with darker part position is at the top (to block the bright sky at certain number which is smaller than 100% ) and the translucent part position is at the bottom (to allow 100% the light from the ground object like the buildings, lake, tress or other objects). So the result of the photos will become different brightness with the actual is. The different is the brightness balancing of the sky part (which looks darker as the light at the photos become less than 100% of the actual light) vs the ground side (which looks brighter as as the light at the photos = 100% of the actual light). Of course we also can adjust the filter position and filter bock area portion by rotating and sliding the filter up-down to get what we want ;p
