About ND filters'ND' refers to 'neutral density', meaning that the filter reduces all wavelengths (i.e. colours) from passing through in equal amounts. Looking through the viewfinder the photographer sees a darkened image with no particular colour cast. ND filters are available in various strengths. Below I consider the effect of plain ND filters, rather than the graduated ones (which are used when you want to darken a particular part of the image, such as the sky). More info in the Wikipedia entry.
Stops, light reduction, filter numbersI've always found it difficult to grasp the concept of 'stops' of light. When looking at various ND filter strengths I was getting confused by how much a stop was, when referring to shutter speed. The numbers below explain how to figure out what effect a given ND filter will have on shutter speed.
Daylight conditionsOn a recent time-lapse shoot in changeable daylight conditions, I found when shooting in Aperture Priority mode at f/8, the shutter speed would vary from 1/640 in bright sun to 1/320 when cloud passed over. Afterwards I considered what effect on shutter speed and whether it would introduce any motion blur.
ND filter numbersHoya and many manufacturers quantify their filter strength by using a set of numbers that double: 2, 4, 8, 16, etc. Tiffen uses a different scale of numbers: .3, .6, .9. I was researching the Hoya style when I put together the numbers below.
- ND2, ND4, ND8, etc refer to how much light passes through, as a fraction. So ND4 allows 1/4 of light to pass through. ND16 allows 1/16 through.
- If you test your exposure without a filter, you can calculate what effect the ND will have. In my example above, an ND8 filter (in bright conditions, exposure normally 1/640) will slow the shutter to 1/80. ND8 allows 1/8th of the light through, which would extend the shutter time by 8 times: 640 / 8 = 80. In the lower light conditions, normally 1/320, with the ND the shutter speed would be 1/40: 320 / 8 = 40.
- If you want to measure the effect of the filters in the units of stops of light, this is also easily figured out: 2 to the power of [stops] equals the ND number (and therefore division of light), for example ND8 is 3 stops: 2^3stops = ND8. ND32 is 5 stops: 2^5 = 32.
In practiceWhen shooting you need to know what the normal shutter speed for exposure would be. Then you can calculate what effect a given ND filter would have on the shutter speed. You're unlikely to have a vast array of filters so you'd have to make a judgement call on what is to hand.
What if you want to hit a target shutter speed? Say you want a shutter speed of 1/2 and you only have a ND16 filter. From the calculation above you can see that normal exposure should occur at a shutter speed of 1/32, which is quite slow for daylight. Perhaps you could achieve that with a tiny aperture, or if you were exposing for a dark area of the scene and didn't mind over-exposed areas elsewhere in the picture.
Maybe you're shopping for an ND filter to use in daylight conditions. Using the same f/8 aperture as above, you'd need the equivalent of ND320, something you can't buy off the shelf. There are ND128 filters available, but in square sheet form rather than screw-in filters, so you could use two of those stacked together (equivalent of ND256) and close the aperture slightly. However using multiple filters can introduce ugly reflections between the elements if the sun catches in a certain way. And using very strong ND can introduce a colour cast, which is more prevalent in cheaper items.
I hope this guide is useful. If you are looking for professional film making services (including time-lapse shooting of any duration from one day to several years), see the website for my new company: Construct Films.