I've used B+W F-PRO 007 Neutral MRC Filters on all of my lenses since going digital. I also use a B+W F-PRO CPL on occasion, mostly for getting a slow shutter speed for waterfalls.
Below are a few tests from this morning, all @400mm. I include the CPL, even though I wouldn't normally use it for these situations. An earlier waterfall photograph below.

Sign in a kids playground

Mallard Hybrid, Anas platyrhynchos

Pink Rock Rose, Cistus albidus
I've also used a Nikon 6T Achromat on different lenses for 30 years. I can't test as above, but here is an an example using in on the 100-400mm. The Neutral filter is removed:

Scarlet Sage, Salvia Coccinea
And here is a waterfall example with the B+W CPL:

Small Waterfall in a Local Wildlife Habitat
- Richard
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http://www.rsjphoto.net