It's a clear filter, and as Radiating stated, it's part of the optical formula of the lens. It's actually designed to hold a gelatin filter (cut to size), although with digital the need for color correction is moot. I suppose one might want an ND gel in the 600 II to shoot Yosemite Falls from Merced

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So...leave it in, unless you replace it with the drop-in CPL when you want polarization.