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Exposure: Shutter Speed and ND Filters

Whereas in photography shutter speed is a parameter that can be adjusted to set exposure, in video this is not adequate, because shutter speed has to be used to control motion blur.

In general, shutter speed should follow, approximately, the 180 degree rule: the shutter should be open half of the time between one frame and the next one. That is: if you shoot 24 frames per second, shutter should be approximately 1/48s (usually 1/50s or 1/60s); if you shoot 60 frames per second (for slow motion), then shutter speed should be approximately 1/120s (usually 1/100s or 1/125s).

If you use a slower shutter speed (e.g. 1/30s when shooting 24p) you get more light in, but motion becomes too blurry. If you use a faster shutter speed (e.g. 1/100s when shooting 24p), motion will be overly strobic, and you'll get an image with too much adrenaline (this kind of effect is used, for example, in the starting sequence of Saving Private Ryan, and in fights in Gladiator; for most footage, it is not adequate).

The reason for this rule to be approximate is that there's a more important one: if you use artificial lighting, you'll probably have to set shutter speed at a specific value in order to avoid flickering and rolling light bands (example). In PAL countries, use a multiple of 50; in NTSC countries, use a multiple of 60; and in any case check your footage to confirm that it doesn't have these issues (you don't want to end up being forced to throw away your work).

Given that aperture is used to control depth of field, and ISO doesn't have enough range to control exposure on its own (e.g. few cameras offer ISO values below 100), we need another tool: neutral density filters (ND), which are black filters of different intensities that aim to take away part of the light without affecting image quality (sharpness and colors).

There are various notations to state how much light is blocked by an ND filter:
scale 1scale 2% lightstops
ND2 ND 0.3 50% 1
ND4 ND 0.6 25% 2
ND8 ND 0.9 12.5% 3
ND16 ND 1.2 6.25% 4
ND32 ND 1.5 3.125% 5

On a bright day, in order to shoot at ISO 100, f/2.8 and 1/50s, I usually need about six stops of ND filtering (e.g. 1.2+0.6 or 0.9+0.6+0.3).

There are several kinds of ND filters: screw-in, for Cokin system, for matte box, faders, etc. I currently use screw-in ND filters, as they are the ones that offer good image quality at a reasonable price. You can check my tests with different kinds of filters, plus additional comments on the advantages and issues of each one, here.

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