I have a Tiffen variable ND filter. I was wondering if the quality is good enough for video, since I'm pretty new to this. I've heard people using other filters but they are way too expensive for me right know, so I was wondering if, quality-wise, the Tiffen is a decent filter for DSLR videomaking.
If a filter is good enough for photo, it is good enough for video. Video is much MUCH lower quality than photography. Ultimately, it really depends on your standards though and we can't tell you if it is good enough for your particular tastes and needs. The reason that there are more expensive cinema lenses is because of how focus works while zooming.
Bob, if you ask 10 ASC Cinematographers which filters they think are best and which lenses are higher quality, youll get 10 different answers.
Point is, it is subjective. You have to test and choose for yourself.
Frame a nice picture, with dark and light elements in frame, with a bare bulb in frame, add something highly textured (like plants or a piece of clothing), and with some skin tone (a model). Then shoot some tests with your lenses, with and without the ND filter, with the lenses wide open and also with your most common shooting stop (f/2.8 to f/4 probably), and screen the results. You will clearly see if your ND filter flares, or has a color cast, or makes you lose resolution, or not. And then you can decide if you are ok with it or not.