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Using ND Filter to create special effects

Sudipta from Pleasure Photography tells us how we can use ND Filters in certain areas of photography where you are looking for a specific type of effect under adverse lighting conditions.

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Article by Sudipta from Pleasure Photography.

ND (Neutral Density) Filters are useful in certain areas of photography where you are looking for a specific type of effect under adverse lighting conditions.

Let me break that down for you: What do you do when you go out on a sunny day and you know you are going to spend a lot of time outdoors? You get a cap and sunglasses. That's right, you want to counteract the effect of direct sunlight so that you are not 'blinded'.

Set a slower shutter speed to get that 'motion blur' in flowing water thus giving the dreamy silky effect.
Cameras work in a similar way, except we aren't 'blinded' by continuously looking at a moderately lighted scene. Let me explain more on camera terms with an example.

Say you are taking a photo of a waterfall and you want to have that 'creamy', 'silky' look of flowing water.

The enemy of a slow shutter speed is the ambient light. Just try this experiment: Put the camera on a tripod in P mode. Making sure the flash is off, point your camera towards any object and press the shutter half-way and record the shutter speed and aperture setting which is automatically selected by the camera. Now change the mode to M and select the same shutter and aperture. Take the photo with this setting, it wont come out that bad. Now the fun part, slowly reduce the shutter-speed one by tenth of a second at a time and see how the photo looks. You will notice the photo is becoming brighter and brighter and at a point everything is so bright that nothing can be figured out clearly. Pros refer to this condition as 'blow-out'.

An example of 'increasing the exposure' by reducing shutter speed.
But say you need slow shutter speed to get that motion blur. Of course, you can reduce aperture but how much? f/16, f/22, f/39? There will be a situation in broad daylight when you have the aperture set to the camera supported minimum, and still you are not able to lower the shutter speed enough to get that motion-blur without 'blowing-out' the photo.

You want to reduce the amount of light reaching the camera sensor, so that the photo does not blow-out and yet you can comfortably reduce the shutter speed to achieve the motion-blur and at a decent aperture setting.

Ladies and gentlemen, welcome the ND Filter. This acts like sunglasses on your eyes, limiting the amount of light that reaches the camera sensor or 'eating' up light so you can take photos in broad daylight and still set the shutter speed as slow as 1sec at a reasonable aperture of f/22.

Standard settings I use for silky/creamy water flow effect putting the ND filter on:
  • Mode: Shutter Priority (Tv)
  • Shutter Speed: start with 1/10 secs and reduce it gradually till you get the desired amount of blurriness effect
  • Aperture: f/13-22
  • ISO: 100 or lower
  • White Balance: AWB or Cloudy
  • Metering Mode: Evaluative (if I have bright area in the scene, I do a partial metering)
Consider another situation, where you want to focus only on the subject keeping the background out of focus, so you widen the aperture to the smallest value your camera/lens supports (say f/2.8) and guess what, the photo becomes 'blown-out'. You try to increase the shutter speed, but alas, even the maximum supported shutter speed is producing a 'blown-out' photo. You guessed it right, the ND filter is again to our rescue. Put on an ND filter and you can keep the minimum f-stop with a reasonable shutter speed and still get the subject in focus with rest of the scene out of focus (I am deliberately avoiding the concept of depth of field to keep this article simpler).

Of course, you can try lowering the sensitivity to ISO50 or lower, but the effect of reduction of light, by lowering ISO sensitivity is very negligible compared to ND filters. What I mean is lowering ISO is no match against the ND filters in reducing the impact of 'amount of light' on brightness of the photo.

Types of ND filter
There are different types of ND filters available and are classified based on the amount of light it blocks (or the darker/denser the glass is). The greater the optical density, the more light it will absorb. So an ND filter is sometimes classified in terms of density: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 2.0, 3.0, 4.0 and so on.

The light blocking capacity of the ND filters is also measured by the reduction of f-stops. The more f-stops a ND filter will reduce, the less light it will allow to pass. Therefore a 2-stop ND filter will block double the amount of light than a 1-stop ND filter.

So this is one way of specifying the 'darkness' of the filter. There is another.

Different manufacturers use different conventions. ND4 filter means a 2 stop ND filter. ND2 means 1 stop ND filter (2=2 to the power 1) allowing 50% light to pass (transmittance), ND4 means 2 stop ND filter (4=2 to the power 2) allowing 25% light to pass and ND8 means 3 stop ND filter (8=2 to the power 3) allowing 12.5% light to pass and so on.

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 2.0 3.0 4.0
by f-stops
1/3 2/3 1 1 1/3 1 2/3 2 2 1/3 2 2/3 3 3 1/3 6 2/3 10 13 1/3

So, 0.3 ND filter = ND2, 0.6 ND filter = ND4, 0.9 ND filter = ND8 and so on. If you are a little bit lost, do not worry, all you need to understand is that 'darker' ND filter will block 'more' light. ND8 is darker, ND2 is lighter. 0.9 ND Filter is darker and 0.3 ND filter is less dark, 3 stop ND filter is darker and 1 stop ND filter is less dark and so on and so forth. That should work for now.

Which one to use and when?
The ND Filters you need will depend on how much light you want it to absorb you can achieve the desired shutter speed or the desired aperture size at the ambient lighting conditions (remember the first statement in this article?). Hence this depends on how much ambient light you are working in. So overall this is more or less experimental. The rule of thumb is - if you want a lot of motion blur or absolute silky-ness, use the darkest ND filter (ND8) so that you can really slow down the shutter. If it's dark (overcast, dawn or dusk) you may not need the darkest ND filter because already there is less light. So you may try a medium dark one (ND4) to achieve the same effect. For sports, to bring that motion blur, you may need just a slight dark one (ND2) if  you don't want too much blur.

A waterfall example
Do I really need ND filters?

You will most likely need ND filters (of various strengths) if you shoot landscape a lot (like me, which you can see in my photoblog, 75% of my best collections are landscapes). Or you shoot sports a lot in bright daylight. But as I said, you will know you need a ND filter when you have reached your camera/lens limit of blocking amount of light and do not have any further option.

Bonus tip: You can always 'stack' up one filter on the other to increase the 'darkness' even more. But beware of vignetting at wide angle shots (18-20mm) with stacked up filters along with other 'combination' effects.

How many do I need to buy?
In my opinion get hold of a 0.9 ND Filter first and then if you need to, go for a 0.6 ND Filter. Then you can stack them up to get even darker ND filter. I have rarely used my 0.3 ND filter until now.

When not to use the ND filter
A word (or sentence) of caution: Most ND Filters are effective only on the visible spectrum of light and do not proportionally reduce ultra-violet or infra-red radiation. This can be specially dangerous if you are using ND filters to view sources like sun or white/red hot metal or glass which emit intense non-visible radiation which are not blocked by the ND Filters and can seriously damage your eyes as the source does look dim when viewed through the filter. Do not look directly at sun through the viewfinder even with a ND filter. You eyes are precious, especially if you enjoy photography.

Another situation to avoid using ND-filter is when the scene has a mixture of areas with higher and lower brightness (i.e. not uniformly lit). For example, during a sunset the horizon is bright but the ground is dark. Using a ND-filter will make the darker spots black, losing the appropriate detail (this is opposite to 'blown-out' and is called 'burn-out').

What are the available varieties? Which one to choose?
There are different flavours of ND Filters that are available in the market. To start with I would always suggest to go for a 'Multi-coated' ones as they are better quality than a normal glass and worth the price. The normal glass ones are cheaper and has a lot of side effects (coloru-casts) associated with it. Of course there are 'Pro' ones that cost a lot, but then they are durable, scratch resistant and best quality.

Options include different brands such as Hoya, Single Ray, B+W, Tiffen and Lee filters. These are great filters with no colour casts but the cost varies with make and model. I own Hoya filter sets and I am quite happy with the quality of the light reduction and at a reasonable price. Single Ray filters are relatively very costly but with high optical precisions. In my opinion, for experimental learners, I would recommend getting a hands on Hoya filters.

Why are these sunglasses for camera called 'Neutral'?
Good question. Because these sunglasses (should) 'eat up' light of all wavelengths equally. This means during absorbing, no colour is given preference over the other, hence the term 'neutral'. But not all ND filters in market are made perfect. Specially the cheap ones that create coloru casts in recorded photos as they cannot reduce intensity of all wavelengths equally. So I would recommended getting standard, branded and quality ND filter (the multi-coated ones). Research on the internet, read reviews and forums to find out the best ND Filter that will suit your needs and your pocket. If you would like to read my suggestions, you can visit the Pleasure Photography website.

That's all for an introductory article on ND filters. Get one and enjoy your photography.

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