TechSupport
Neutral-density filters for video?
QI recently upgraded to an Olympus OM- D E- M1 Mark II from my trusty old E- M5 and would like to start exploring video. I know next to nothing about video and have only done a few recordings to play with the settings.
During the recent hot and sunny weather, I noticed that if I wanted to maintain a nicely blurred background – as I do with stills portraits, for example – with a bright aperture, anything moving moderately fast started to look rather jittery. Could this be something to do with the bright conditions, and would a neutral-density filter be beneficial? I have to confess I haven’t yet found a need to use filters like these for my stills photography so far. Ben Fakenham
AYou’re absolutely on the right track. Normal perception of motion is a function of the brain in conjunction with the human eye. We see and expect to see things that move with natural motion blur. For still photography, you can choose to show motion blur or freeze the action according to your artistic imperative. To freeze motion you need comparatively short exposure, achieved using a fast shutter speed or something like the short duration of electronic flash. It’s more complicated with video. You can think of video as being a fast succession of still frames. This can be 12 frames per second (fps) for classical stop-motion animation; 24fps for most Hollywood movies; and 25, 30, 48, 60 and higher frames-per-second rates variously favoured by videographers for their own purposes.
Despite the various frame rates used, one thing that is almost universally agreed is that if you want to produce natural-looking motion in your movies, you need to aim to match the shutter speed to 1/x sec, where x = double the frame rate. It’s also referred to as the 180- degree rule, which is a bit anachronistic as it refers to old-fashioned rotating shutters on old movie cameras – but the rule itself stands to this day. If you keep to the rule and your video frame rate is 24fps, your target shutter speed should be 1/2 x 24, or 1/48sec. For a frame rate of 60fps the shutter speed should be 1/120sec, etc.
If the shutter speed is faster than the target, you risk introducing unnatural and even jerky action. If the shutter speed is too slow, you risk smeary-looking action. The E- M1 Mark II can do 24fps, widely acknowledged as the ideal movie frame rate, so your target shutter speed is 1/48sec. In normal daylight, you’ll need to stop down the lens to keep the shutter speed that low. If you want to use a wider, brighter, aperture, you need to reduce the light transmitted through the lens by another means and that’s where neutral- density filters come in.
For example, if the shutter speed without a filter is 1/800sec and you need to get it down to 1/50sec (close enough to 1/48sec), you’d need an ND16 filter to reduce the light by four stops/EVs, down to 1/16th of its unfiltered brightness.