Experiments with a DIY Variable ND
Variable-ND filters, sometimes called fader ND filters, are especially handy for video use since the cinematographer is sometimes bound by shutter speed, frame rate or sometimes ISO. For instance, the scene might be bright outdoors with a shallow DOF treatment, but the iso is already at the lowest and shutter speed must be at 48 or 50 when at a 24 frame rate.
Being able to dial in the specific exposure with a vari-nd would be the fastest way to achieve this than fiddling around with several ND filters. The example screen grab from my tests show this scenario, shot in bright mid-day sun at f2 with a shutter of 47. This was with a Nikkor 50mm f1.4.
Variable NDs vary in price, from 100 to more than 500 US depending on ring size and brands. Some are better than others.
To those who want to experiment with its benefits without spending, and if you got old polarizing filters lying around, you can make a DIY "Vari-ND" filter, note the quotes, by stacking a linear pola over a circular pola filter. Or, if you like its effect and find it useful, you can rehouse the circular pola in another filter ring that does not rotate to make it convenient to use.
So, does it work? Well, if mated to the "right" lens, it does. I tested several lenses, I used the following, Nikkor 50mm f1.4, Nikkor 105mm f2.8, Nikkor af 80-200mm f2.8. The wide lenses I have would vignette so I did not include them. Draw your own conclusions after watching the tests I did using color and resolution charts.
Hope you find this post useful.