B005VLI4QA Hama 55mm Variable Neutral Density Filter – Black

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Enables exposure to be lengthened between +1 and +8 aperture stops
With rotating front lens
For reducing light if the brightness is too high
Allows the photographer to use large apertures while taking pictures outside = backround gets hazy
If exposure times longer than 1/30 seconds, water flows (if shorter, it freezes to ice)