ND filter for Raven - strengths needed?

[Brandon Shafto]

Grab a matte box and invest in a 4x4, 4x5.56, 6x6 set.
These look promising: https://vimeo.com/142476661

http://www.bhphotovideo.com/c/searc...|1&setNs=p_PRICE_2|1&N=0&srtclk=sort&Ntt=nisi

 

[Jacin Lowe]

I can wholly attest to the Nisi Line of ND filters.
I bought the 4x5.6 1.5 IRND from them and i have seen zero color shifts what so ever.
and the great part is they are only $250 for for each in the line.
They only go up to 2.1 IRND sadly but i want to try and see if they will mix well with the firecrests'.