Many video professionals prefer to use variable ND filters when shooting with mirrorless cameras or DSLRs instead of the fixed ones for various reasons. For instance, Variable ND filters can make the life on set a lot easier and speed up the overall workflow significantly. You can just choose the aperture you want considering the depth of field effect you’re aiming at, whether it’s a shallower or deeper focus. Whereas with the fixed NDs you need to carry at least three or four of these changing various combinations before you get the right exposure which is not the most practical solution, especially when there is a necessity to move quickly on set.
In some cases, it may be desirable to have the entire image sharp, in others emphasizing the subject while de-emphasizing the foreground and background might be the better creative decision for a certain project. Whatever the case may be, there is no “right answer”. Whichever one is the better solution really depends on your personal requirements and the aesthetics of your creative vision.
The introduction of vignetting and IR pollution is something that many of the cheaper Variable NDs suffer from as well, so the upfront side-by-side testing is still one of the most reliable and convenient methods to make the right decision. The following test conducted by the Sabi company did a quick comparison between the SLR Magic Variable ND and the Tiffen Variable ND.
Variable ND Comparison: SLR Magic vs. Tiffen from The Sabi Company on Vimeo.
Many would expect that the Tiffen Variable ND next to the SLR Magic Variable ND would produce similar results with minor differences. However, this test shows that this is not the case. The main difference is the Tiffen appears to be affected heavily by infrared light (the so-called IR pollution) producing a muddy, brownish quality that’s altering the image drastically from how it would appear without any filtration. The SLR Magic filter, on the other hand, is adding a tiny bit of warmth to the image much lesser degree compared to the Tiffen. The test was shot with the Panasonic GH4 in 4K with the Natural profile, set with an ISO of 200.
As expected this colour shift will affect mostly skin tones of your talent which is another reason to be extra cautious when using ND filters for commercial work. The polarised effect that occurs when using ND filters can create an unnatural look that can be avoided by using dedicated fixed ND filters in particular strengths. The polarizing effect is something that is typical for the majority of the variable NDs due to the way they are made.
As-a-rule-of-thumb it’s always recommended to test and compare a bunch of filters before you make your final choice.