Ian R. Brown wrote:
ND filters probably are the best solution but there are a great many complex shooting variables that can make them a PITA to someone like me who is new to their use. So the idea of being able to add blur at leisure, over a coffee etc., is very seductive.
Sorry no one else had jumped in to correct me or to give you better advice. This is unbelievably complex software, huge computational and rendering resources using motion tracking at a pixel level and motion prediction in order to create blur—and to get the directional blur correct—where only razor sharp edges exist.
The thing about neutral density filtration is it's been around forever. It's predictable, reliable, and inexpensive. Variable NDs are convenient but the polarization might be unwanted. The section on ND filters in every "how to shoot killer video with your DSLR" book is readily understood, the ND factor is a denominator; ND2 is half the light (one stop), ND4 is 1/4 light (two stops) &c. You need 6 stops (ND64) to go from 1/2000 to 1/30 second shutter speeds. That's going to be two NDs stacked, a 16 and a 4 or two 8s.
From around the web:
here are two common ways of quoting ND filter strengths, and one less common:
- 2x, 4x, 8x, etc. Sometimes these are referred to as ND2, ND4, ND8, and so on. These refer to the amount by which the light is diminished. An ND2 filter halves the light, while an ND8 filter reduces it to one eighth.
- 1 stop, 2 stops, 3 stops etc. Sometimes these are referred to as EV, for exposure value. These are probably the most convenient measurement because they tell you how many stops they'll adjust your exposure by.
- Numbers like 0.3, 0.6, 0.9 etc. These are basically just 0.3 x the number of stops of EV. These are less common.
Each stop of exposure value refers to a halving of light, so:
- 1 stop = ND2
- 2 stops = ND4
- 3 stops = ND8
- 4 stops = ND16
And so on.
Stacking multiple ND filters adds stops, and multiplies strength values.
So, ND500 sounds like a lot, but it'd be the same as stacking an ND16 and an ND32 (16 x 32 = 512; manufacturers round it to 500).
The main disadvantage of neutral-density filters is that to be entirely flexible in your shooting you need to carry a range of different NDs. This can become an expensive proposition, especially if using screw filters with different lens filter sizes, which would require carrying a set for each diameter of lens carried (although inexpensive step-up rings can eliminate this requirement). To counter this problem, some manufacturers have created variable ND filters. These work by placing two polarizing filters together, at least one of which can rotate. The rear polarizing filter cuts out light in one plane. As the front element is rotated, it cuts out an increasing amount of the remaining light, the closer the front filters comes to being perpendicular to the rear filter. By using this technique, the amount of light reaching the sensor can be varied with almost infinite control.
The advantages to this are that you get multiple ND filters in one package, the disadvantage is a loss of image quality caused by both using two elements together and by combining two polarizing filters.
(