| http://www.w3.org/ns/prov#value | - never clear to me...but they worked about the same and I never inquired further.Then there was a discussion here about the use of ND filters in doing times lapses, and I found VARIABLE ND filters for sale...apparently using two polarizing filters variably angled...but they were a significant cost....and I already had PF's on every single lens I own.Sooooowhy not stack two and angle/dial them to g
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