about GND -- from 雲海飛雨

This is a reponse post from 雲海飛雨. It has very good information about GND. Repost it here and hope other beginners like me could see it. Thanks a lot 雲海飛雨 !

(Don't know what happened, I could not type in chinese this morning in my pc.)

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http://purewater.hep.com.cn/upload/filter/gnd.pdf

靜心大師在 << [純技術討論]精確曝光-你完全勿須做後期>>中 苦口婆心指出:

很多時候我不理解朋友們可以花上千刀去買鏡頭，為什麽不再花個百刀去配上必需的濾鏡。要知道任何專業攝影師的包裏，不一定有某些特殊的鏡頭，但卻是一定有一大堆各種濾鏡的。