Fotodiox Pro 186mm スリム ND16 フィルター - ND16（4段）フィルター（WonderPana XL システム用）

I was worried the ring might feel flimsy at 1mm thick, but the anodized aluminum is surprisingly solid and the glass itself is flawless. It cuts exactly 4 stops of light with no color cast, which saved my wide-aperture shots on a sunny beach.

Finally, an ND filter that doesn't create a color cast on my wide lens. The 186mm slim ring fits perfectly into the WonderPana system without any vignetting, even at 14mm. Really impressed with the sharpness from the optical glass.

The glass is crystal clear and the slim 1mm ring slides onto the WonderPana without any vignetting on my wide lens. It handles the 4-stop reduction exactly as expected, though the threading could be a touch smoother. Still, for the price, it beats the cheap resin alternatives hands down.