We know that filters degrade image. The longer the focal length and the bigger the aperture of the lens, the more degradation. Filter quality (flatness) plays big role. If a filter is used in front of say 400mm lens, then even a supposedly high quality Hoya HMC Pro will cause very significant (unaccaptable) image degradation. This is because the filter needs to be flat to something like 1/2 light wavelength, so even placing a filter in a frame will buckle it beyond that! Scarry stuff!
1. What filters (in particular NDs) are the best quality?
Pancro, Schneider, Formatt,... they all say they are "flat" but how flat is flat?
2. What is the maximum focal length and aperture one would use a filter in front of the lens for 4k acquisition?
Of course internal filters are the answer, but show me a lens with 3 internal filter frames....