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A good website to see what company offers what stop for ND filters. http://www.2filter.com/4x4camerafilters/4x4ndgraduals.html
I chose schneider because it's glass, I like the weight of it, doesn't scratch as easy, i like the images i'm getting, easy to clean, and the 4x4 was easy enough for me to hand hold in front of a 16-35 lens without vignetting and no fingers in the photos . I have the ND1.2 because ND0.9 wasn't strong enough for me. Despite saying that, I had to get the 4mm guard rails for the Lee foundation kit holder because the schneider glasses are 4mm thick as opposed to Lee which are 2mm. The singh-ray filters are also 2mm. It is easier to hold the 4x6 in front of the lens though. If you can, you should borrow a 4x4 and a 4x6 and see which one you like more for what you do. I originally hand held my filters but will be switching to a holder now to decrease my setup time and to hold at least 2 filters.