You use a neutral density (ND) filter for the purpose of cutting the light down therefore requiring the shutter to be open for longer.
This has the effect of blurring motion (among other things) and is often used for making silken effects from moving water or clouds.
Different brands of ND filters use different naming conventions* to describe their effect.
Now let’s say you put an 8-stop ND filter on the lens (e.g. a Hoya ND200, or a Haida ND2.4). This filter cuts the light down by 8 stops or 1/256th. It means you will have to reset the shutter speed by 256 times to compensate.
In our example, you would need to reset the shutter speed to 1/100s X 256 = 2.56 seconds to get the same exposure (amount of light) that you had without the filter.
A common use for an ND filter is to ‘stop’ flowing water. That normally means using a shutter speed of 10 seconds or more — a so-called ‘long exposure’.
Suppose you were photographing a river and without a filter the scene was correctly exposed at ISO 100, f/11, and 1/50 s. If you put a 10-stop filter on the lens, you’d cut the light coming in by 1/1024 so you would need to adjust the shutter speed to 1/50 x 1024 = 20.48 s (say 20 s)
While the multiplications to do these calculations are not difficult, there are smart phone apps that will do all the calculations for you, or even tell you what filter to use to produce a particular shutter speed. Go to your app store and search for ‘ND filter’. The apps are usually not expensive (after all, they only have to multiply and divide by 2 a few times!)
Different shaped filters
ND filters come in two flavours: screw-on or slot-in.
Screw-on NDs simply screw onto the front of your lens like any other filter.
Slot-in filters come with a little kit that places a holder at the end of the lens and the filter is slid into the holder.
There are advantages and disadvantages to both. The slot-in filter systems are a bit fiddly to set up initially, but changing filters is very easy. Slide one out and slide another in. Whereas screwing a filter on the front of a lens is simple. On the other hand it takes a bit longer to change screw-on filters if you need to do so.
There’s another minor disadvantage of a screw-on filter. If you are using a dense filter such as a 10-stop, it is often necessary to focus the lens before putting the filter on because you can see nothing at all through the viewfinder once the filter is in place. It takes a bit of practice and a lot of care to screw on a 10-stop filter without moving the lens and upsetting your focus. Sliding a slot-in filter into its holder does not risk refocussing the lens.
ND Filters are commonly made of a resin material. Some are made from glass. Glass ones are more fragile (obviously) and typically more expensive.
ND filters should do nothing more than cut light without changing colours. But the dyes used, particularly in resin filters, are notorious for introducing colour casts: some brands introduce a cool blue colour cast, others a reddish tone. These effects are most noticeable in dense filters such as 10-stop ones where more dye is used in their manufacture. It is usually possible to remove the cast in post processing just by playing about with the white balance controls. The major issue with cheaper brands of ND filters is that these colour casts are not uniform across the filter which makes correcting them a nightmare.
Higher quality (usually glass) ND filters are generally quite neutral.
The best filters are not cheap. On the other hand price alone is not the best indicator of quality: some of the most expensive ND filters introduce significant color casts. A number of members have a lot of experience with ND filters. If you’re thinking of buying ask around. Some brands offer very good value for money.
Here’s an example: I shot this at ISO 50, f/11 and the shutter speed was 1/100s
Then I put on a 10-stop filter and the shutter speed needed to be lengthened to 10s
Of course if you are looking to make a shot with an exposure of 10 seconds or more you will need to have your camera on a tripod unless you are a statue!
*PS: Extra for experts! Knowing or not knowing the following will make no difference to your ability to make good photos with an ND filter. It might make you a more boring person at dinner parties though!
The way different companies name their filters usually depends how they measure and describe the light falloff/absorbtion.
Each stop halves the amount of light. So some filters are described by how many times the light has been halved. In this system a 3-stop ND filter would be called an ND3 would have halved the light three times to one eighth of its original strength.
½ x ½ x ½ = ⅛
Another system would describe the falloff in terms of the fraction of light getting through (compared to no filter). In that system a filter of the same strength as the one above would be called an ND8 because it divides the amount of light getting through by 8. (Cokin filters are named this way)
A more complicated system recognises that the light falloff is exponential but instead of counting halves they count powers of 10. Maths students will know that 10^0.3 ≈ 2, and 10^0.6 ≈ 4 and so on. Conversely, log(2)≈0.3 log(4)≈0.6, log(8)≈0.9 etc. In that system the strength of the filters is measured by the log of the amount by which light getting through is divided. So a 10-stop filter which lets only 1/1024 (≈ 1/1000) of the light through would be called an ND3 (or ND3.0) because log(1000) = 3. Tiffen, Formatt-Hitech, and Haida all use that system.