Lee filters were traditionnally a high-end brand of filters. But since Chinese brands such as Haida and Nisi have developped into the market, the old brands like Lee and Cokin aren't necessarily the best anymore. Haida (Nano Pro and Red Diamond 150mm) and Cokin XL (Nuance extreme 130mm) are now some of the most color neutral filters available, although Cokin filter holders are easy to use but introduce more vignetting than other brands of filter holders. IMO, Lee have fallen behind, as well as Format Hitech not colors neutral at all. Haida filters are the most color neutral I've found. Filter holder is key to have filters easy to insert and remove, while still having filters sufficiently held in place.
Originally posted by Medex before buing filters try composite mode on Pentax K-1 (if you own this camera, maybe other have that option too).
That's an idea, although without filter at all, one may end-up stacking 200 images to produce a 2 seconds exposure. At least, a few stops of ND is good to have to avoid stacking hundreds of images for one final image.