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B&H Photo Video has Tiffen 77mm Variable Neutral Density (ND) Filter w/ 2 to 8 Stop Light Control for $78.95. Shipping is free.
*Save an additional $10 when you checkout via Chrome Browser for Android mobile devices using Google Pay (more info) . Shipping is free.
Editor's Notes & Price Research
This filter allows you to have continuous control over the amount of light coming through your lens in an approximate range of 2 (ND 0.6) to 8 (ND 2.4) stops - while maintaining the integrity of your image.
$80 + free s/h