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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?

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Please note: this is a question I'm often asked so I'm intending to answer it myself, as per meta.stackexchange.com/questions/17463/…. Just thought I should point that out before other people spend time on their answers! –  Mark Whitaker Aug 28 '11 at 20:09
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per the site FAQ even - ' It’s also perfectly fine to ask and answer your own question, as long as you pretend you’re on Jeopardy! – phrase it in the form of a question.' –  rfusca Aug 28 '11 at 22:08
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Not to mention we're kinda used to it here - @Jay went on a big Jeopardy streak there for awhile. I was going start calling him Ken Jennings... –  rfusca Aug 28 '11 at 22:09
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Two great answers already, but I'd also wish to hear: (1) How the NDness is achieved, how they reduce the captured light, what are the filters made of? (2) What is the common price range (in such a precision that the casual browser would know if he/she should reserve tens or hundreds of $/€). –  koiyu Aug 29 '11 at 9:20
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@koiyu - good points - I urge you to go out there, do the research and add an answer! –  ysap Aug 29 '11 at 10:27

3 Answers 3

A Neutral Density (ND) filter is a filter that reduces the amount of light captured by the camera evenly across the visible spectrum. As such, it looks grey to black (depending on the filtration power) and does not cast color on the received image (like blue or yellow, e.g., filters will do).

When using a ND filter, there is a need to compensate for the reduction in light by using longer exposures (or higher ISO, but it usually defies the intention of ND filtration). This way, one can capture long-exposure images that are not possible w/o the filter as the camera (set to its lowest ISO and possibly smallest acceptable aperture) is at its slowest speed for a good exposure.

Examples for such situation is when shooting waterfalls in a fairly lit location. Then, the speed for correct exposure is not slow enough to create the required streaming water effect. Using a ND filter, one can reach slower speeds, as if the scene lighting was dimmer.

Note that a uniform ND filter does not change the Dynamic Range of the scene, as the bright areas get darker in the same amount as the dark areas.

Another type of ND filters is the graduate ND filters. These filters usually attenuate the light only at one half of the frame and gradually get clearer on the second half. These filters are used when shooting landscapes, where the sky is much more bright than the scenery and using good exposure for the ground will overexpose the sky. A graduate ND filter lets one reduce the brightness of the sky and get it captured nicely with the ground.

ND filters are marked in multiplies or stops. An 8X ND filter is similar to ND3. Both cut 3 stops of the light. If the camera's meter tells that 1/500 sec is required for the given aperture, then using a ND3 filter will allow an exposure of 1/64 sec with similar outcome.

There are some (expensive) variable ND filters. These are made from two polarizers, where the front element rotates and thus let you set the amount of light being cut (up to virtually no light passing through).

The image below, from Wikipedia, shows the effect of an ND filter:

enter image description here

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Using larger aperture or more lighting can also be used to compensate for reduction of light. Especially in portraiture, ND filters are sometimes used to tame sunlight to get shutter speed into flash sync speed range without resorting to small apertures. –  Imre Aug 28 '11 at 23:29
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@Imre - You are absolutely correct. The ND can be used to shoot large apertures rather than slow shutters. That said, the question specifically ask about long exposures. –  ysap Aug 29 '11 at 0:09
up vote 15 down vote accepted

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.
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No, I never accepted your answer in the first place. See my comment on the question: I always intended to answer this myself and I accepted my own answer as soon as I could. Don't know what's given you that impression. –  Mark Whitaker Oct 31 '12 at 17:55
    
That is odd, seems like a bug in the rep to me then. Check out my rep score history, it shows my answer was unaccepted for this question on Oct 11th - i.imgur.com/refVv.png No worries, I don't care about the rep, I just thought it was kind of funny to see the unaccept. –  dpollitt Oct 31 '12 at 22:18
    
Yep, must be a bug. I accepted my answer on 31 August 2011. :) –  Mark Whitaker Oct 31 '12 at 23:44

You will want a neutral density or ND filter. It essentially darkens all parts of the image giving you a uniform exposure that is darker then it would be otherwise. Depending on the camera, some have a built in ND filter, or some accept a filter accessory that you can screw on to the lens or drop into the filter slot.

Personally I would buy a CPL(circular polarizer filter) and use that when possible since it is dual purpose. It will block some incoming light and allow a wider aperture, but also give some pop that CPLs are known for.

We have lots of questions that cover some more specialized questions on neutral density filters that may interest you. See:

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Great, thanks! I'll need to do some reading before I work out exactly what I need. –  Kirk Broadhurst Aug 11 '12 at 1:10

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