I recently had the opportunity to use Breakthrough Photography's X4 10 Stop ND Filter (round 77mm filter). The company boasts their ND filter is the most color neutral and sharpest on the market. That is a pretty tall position to take when you consider some of the other filter companies such as Lee or Singh-Ray.
My findings were pretty telling. The images above were both shot on Cloudy White Balance as I wanted to ensure the color temperature was constant. Additionally they were slightly finished (edited) identically. The exposure between the individual image comparisons is relatively close with the ND images a little darker.
In the end I could see no discernible difference in sharpness and only the slightest hint of a color shift. The ND produces a little bit of a warming effect on the images, which is easily adjusted in post processing.
So, if you are in the market for some new filters Breakthrough Photography has you covered.
Disclaimer: The Nature Photography Show is sponsored in part by Breakthrough Photography. However, I had already started using their products before they elected to come on as a sponsor.
Should you decide to purchase filters from Breakthrough please follow the link on our website (or click their logo above). This lets them know who sent you and helps out the show in return.