Neutral Density (ND) filters absorb light evenly throughout the entire visible spectrum. In other words, they appear grey (sometimes almost black) to our eyes. When do we use them? When for some reason you need to reduce exposure, i.e. you want to reduce the amount of light that reaches your camera’s sensor.
Why would you do such thing?
For one of two reasons. Either because you want to use very long shutter speeds or to be able to use a very large aperture. The first use is more typical for still photographers, the second is a standard tool for filmmakers. Because of that, I will discuss them in separate posts.
Long shutter speeds in broad daylight
Imagine that it’s a sunny day but you wish to experiment with very long shutter speeds and the motion blur that comes with them. To do that you choose the lowest possible ISO (100 on most current digital SLRs, 50 on some of them) and you make your aperture as small as possible (probably somewhere between f/16 and f/32 on most lenses). Even so, when the sun is out you probably won’t be able to use shutter speeds longer than 1/15 – 1/4 of a second. But what if you want to?
Simple. You put an ND filter on your lens and reduce the amount of light getting through.
Various manufacturers ‘name’ their ND filters in different ways and the result can be very confusing when you’re trying to find out what is it that you need. There are three methods of talking about how much they absorb. You can either specify how many stops of light they absorb. Or, using the scientific logarithmic density scale each stop corresponds to a density of 0.3. Finally, you can state how many times your exposure will need to be lengthened with the filter in question (a horribly confusing and non-sensical approach, who would ever start to calculate how much is 32x 1/8th of a second?).
1 stop – ND 0.3 – 2x
2 stops – ND 0.6 – 4x
3 stops – ND 0.9 – 8x
4 stops – ND 1.2 – 16x
5 stops – ND 1.5 – 32x
6 stops – ND 1.8 – 64x
7 stops – ND 2.1 – 128x
8 stops – ND 2.4 – 256x
I tend to use two filters: an ND 0.9 and an ND 1.5. The first reduces exposure by 3 stops, the second by 5 stops. And if I need to, I can screw the two filters together for a combined 8-stop reduction. (The densities simply add together.)