I can only share my experience. I shoot with a Speed Graphic and an Aero Ektar Lens. The whole point of the set up is to shoot wide open and get very shallow dof. I started by using screw on single nd filters. As I kept using them I decided that wearing out the threads on either the lens or the expensive lens shade I had made was going to get very expensive to repair. Also, to compose a shot, I had to unscrew the filter because it was too dark to focus. I also was finding that to maintain wide open, I often needed 1/2 stop in between full stop values. So I did a bunch of research & went out and bought a mid-expensive Variable ND so I could dial in any amount of stops needed. The biggest problems I came across were the fact that the filter is not marked in stops. This makes it difficult to maintain consistency. Taping it became a pain, too. It also had to be removed to focus and compose. It was also useless at the very low&high end of densities because of the artifacts. They are inherently part of the compromise involved in using that design to make variable ND filters. $200 wasted. I ended up buying a matte box that flips up and holds 100mm square filters. It’s the same old story - a purpose built item is almost always superior to a “do it all” item. They involve compromise(s) that an item built for one function does not.
Yes, good 100mm square filters are expensive. I can see buying mid-expensive filters, but I can’t justify the expensive ones - which are usually 2x the price. I see a difference from the variable nd images to the single density 100mm filters. It is noticeable to my eyes. That’s been my experience. The variable nd seems most valuable to video shooters. For thinks like maintaining aperture when shooting from a dark room to walking out in the sun. The variable nd can be dialed in as needed on the fly.
