What are neutral density filters and how do I use them to create long exposures in daylight?

I've seen a few related posts on the site but nothing specifically on this point:

I'd like to be able to take long exposure photos during daylight (like the examples in this Flickr group). I'm told that you can do this by using a neutral density filter. So my question is: what is a neutral density filter exactly, and how is it used to achieve that smooth long exposure effect?

Answer

Firstly, what is a neutral density filter?

"Neutral density" just means that the filter is a pure shade of grey: it shouldn't (if well manufactured) add any colour tint to your photographs.

There are two main types of neutral density (ND) filters: graduated and non-graduated. Graduated ND filters are darker at one edge and lighter (usually completely clear) at the other. They're typically used to balance out a bright sky with a darker foreground, e.g. in landscape photography. Their usage is dealt with in detail in this question.

For long exposure work you'll typically want to use a non-graduated neutral density filter, to ensure the same effect across the whole picture. In short, the ND filter works by reducing the amount of light coming into the lens, so you can compensate with a slower shutter speed than would otherwise be possible.

Choosing the right filter

ND filters are categorised by how dark they are (i.e. by how much light they block out). There are a few different numbering schemes for describing this darkness which can be confusing to the first-time buyer:

• Number of f-stops: a "2-stop" ND filter makes the same difference to the exposure as reducing the aperture by 2 full f-stops (e.g. switching from f/4 to f/8).


• Shutter speed multiplier: maybe the easiest to understand - a filter described as ND8 (or NDx8) makes the photo 8 times darker, or has equivalent effect on the exposure as using a shutter speed 8 times faster (e.g. 1/160s instead of 1/20s). A ND1000 filter makes the photo (roughly) 1000 times darker, and so on.


• Optical density: Just a different numeric scale, and not one I often use.

Luckily there's a handy conversion table on Wikipedia, and better still you can easily get by ignoring everything but the shutter speed multiplier.

All my daytime long exposures have been taken using a NDx1000 filter.

Taking a daytime long exposure

The steps required are quite easy, it just takes a bit of patience.

• You're going to be taking a long exposure, so start by mounting your camera on a tripod or some other stable surface where it won't move around.


• Compose your shot and start by shooting a regular exposure of it (without using a filter). It usually helps here to switch to Av or Tv mode for better control over the aperture and shutter speed. You should also select a fixed ISO speed rather than using auto-ISO (if your camera has it).

Let's say the camera is taking a well exposed picture at ISO 100, f/8 and 1/100s. Make a mental note of those settings. Now you can start preparing for the long exposure shot.

• Switch the camera into Manual mode.


• Switch the lens into manual focus mode. Once the dark filter is attached the camera may have trouble focusing so it's important to lock the focusing now to whatever worked for the regular test shot.


• Fit the filter to the front of the lens. (Tip: If you're shooting wide angle and have a UV filter on the lens already you may want to remove that first to avoid vignetting around the corners.)


• With the camera now in Manual mode, set the ISO and aperture to the same values we used previously (in my example, ISO 100 and f/8).


• For the shutter speed, multiply the original shot's shutter speed by the shutter speed multiplier of the filter. I'm using a NDx1000 filter so I need to multiply the shutter speed by 1000. So my original shutter speed of 1/100s becomes 10 seconds (1/100 x 1000). Dial that in and you're ready to go! If the camera's auto-metering is flashing at you like crazy, just ignore it. :)


• If possible, use a shutter release cable to avoid knocking the camera during the exposure. Otherwise use a timer mode to fire the shutter automatically.


• As ever, check your results on the camera LCD and tweak to taste.

A few tips

• Using a very dark filter (like a NDx1000) it's possible to take very long exposures (i.e. over a minute). That's when a shutter release cable becomes essential.


• Depending on your filter strength, doing the shutter speed multiplication in your head might be a challenge to your maths! For example, 1/100 x 1000 is easy enough, but 1/60 x 1000 is more difficult. I've written out a little table of common shutter speeds multiplied by my filter strength and keep it in my camera bag. A calculator on your mobile phone can be handy too.


• Despite the fact they're meant to be a pure shade of grey, very dark ND filters (like NDx1000) do tend to add a colour cast to the photo: mine have always turned it slightly purplish. Either correct that in post-processing or embrace it as an effect if it enhances the final result.