As a landscape photographer, I never go anywhere without my ND filters, and these days using them is second nature. However, I often find that my students struggle a little bit with understanding them, so this week's tip is an overview of what they are, why you use them and how to use them.
1 What is an ND filter?
In simple terms, an ND filter reduces the amount of light that reaches your camera sensor, ideally without changing the hue or saturation (although some cheaper filters can give colour casts). They can be circular, designed to screw in to the end of your lens, or square which are attached using dedicated filter holders which attach to your lens.
2 Why use an ND filter?
By cutting out the amount of light that enters your camera sensor, this allows you to use longer shutter speeds. This is essential when there is too much light to create the artistic effect that you are looking for (e.g. smoothing water during daylight hours). ND filters allow you to move from taking 'snaps' of a scene, to begin the journey into photographic art. In the photo below, an ND filter has been used to slow the shutter speed to around 30 seconds, so as to remove all sign of waves on the sea, and give a smooth silky water surface. This has simplified the scene greatly, and the eye is now drawn to the key objects of the composition, rather than the detail of waves.
3 ND strength: the confusing bit!
One of the biggest confusions amongst many of my students is caused by the fact that there are THREE different numbers used to describe the strength of ND filters (i.e. how much light they cut out). Depending on the brand of filter that you buy, one or more of these may be written on the filter, but there is no consistency in which is used! The three different descriptions, and what they mean are as follows:
1) Stops: this is a description of the number of stops of light that the filter will cut out, where a stop is a halving (or doubling) of the amount of light. E.g. a '3 stop' filter will halve the light, then halve it again, then again (i.e. reduce it by a factor of 2x2x2, i.e. a factor of 8)
2) ND number, e.g. 'ND 8'. This describes the reduction in the amount of light, i.e. by a factor of 8. An ND8 filter is identical to the 3 stop filter described above.
3) Optical density (OD). This is the ratio of the amount of light hitting the filter, compared to the amount of light that the filter allows through. An optical density of 0.9 is identical to the 3 stop / ND 8 filter described above.
So, as you can see, it's not surprising that there is plenty of confusion about ND filters, when '3 stop', 'ND 8' and 'Optical Density 0.9' all describe exactly the same filter, and yet filter manufacturers have decided to add to the confusion by all using different ones!
4 Which ND filters do I need?
Cheap ND sets that you might buy from Amazon, often have really low strength (e.g. 0.6 stop, 0.9 stop etc); I find these next to useless, as in almost all situations it's possible to simply tweak your aperture / ISO or shutter speed to get the same effect. I find that the perfect trio of ND filters which allow me to deal with every eventuality are a when shooting landscapes are a 3-stop (ND 8, OD 0.9), a 6-stop (ND 64, OD 1.8) and a 10-stop (ND 1024 [sometimes shortened to ND 1000], OD 3.0). As ND filters can be stacked, this allows me to effectively achieve 3-stops, 6-stops, 9-stops (3+6), 10-stops, 13-stops (10 + 3) etc. For this reason I believe that the 'big stoppers' that are often marketed, are not really required.
5 Round or square filters?
Round ND filters tend to be cheaper, and are fine if you only have one main lens (or your lenses have identical internal diameter screw threads). However, if you have multiple lenses with different diameters, you will need a set for each and this starts getting cumbersome and expensive. For this reason, I use square ND filters. Each of my lenses has a filter adapter (of the appropriate diameter) fitted, and then when required the filter holder itself can be attached and ND filters slotted in. I use the NiSi system, but there are many available; the most common being Lee, Cokin and Hoya.
6 Quality matters
When it comes to ND filters, quality really does matter. The cheaper offerings tend to be made of plastic, and can often create a colour cast, so your resulting photos appear tinged with colour, most commonly magenta. This can be removed in post processing, but is clearly not ideal. More expensive ND filters are made of glass, have better optically quality and should not give colour casts. Although glass ND filters are not cheap (my 150mm square filters are around £120 each) however they should last for years as long as you look after them (and don't drop them as I once did!).
7 How to use an ND filter
Start by setting a 'good exposure' using your light meter and without an ND filter fitted (i.e. an exposure that is not too light and not too dark). Ideally use shutter priority mode or manual mode.
Decide what effect you are trying to achieve; do you just want to blur movement a little bit or a lot? To slightly smooth the water of a waterfall, a shutter speed of around 0.5-1.0 seconds is generally enough, but to flatten the waves on the sea, you generally need a shutter speed of 30 seconds (or sometimes more).
Can you achieve the shutter speed that you want without an ND filter? Often when light levels are low (at dawn or dusk or in woodland) you can close your aperture down or reduce your ISO and get a slow enough shutter speed for your needs; if this is not possible, then you reach for your ND filters.
The first method is to manually calculate the shutter speed you will get when you add a filter; for this, the most useful ND filter description is the 'ND number' described above (e.g. ND 8). For example, if your shutter speed without your ND filter is 0.5 second, and you add an ND 8 filter, then you should be able to increase your shutter speed to 0.5 seconds x (ND) 8 = 4 seconds and still get a 'good exposure'. If this is not long enough, then adding an ND 64 would give a shutter speed of 0.5 seconds x (ND) 64 = 32 seconds when the filter is added.
An alternative to the above is to use one of the many apps which are available which allow you to select a starting shutter speed, and an ND filter strength and they then calculate the new shutter speed for you (in the same way that I have suggested manually above)
The second method (and the one I usually use) is simply to start with a good exposure (using the light meter), then add the ND filter; the photo now looks badly underexposed, so I then simply increase the shutter speed until I once again get a good exposure as shown by the light meter. I find this is the quickest, and easiest method, and you don't need a calculator! The only time this method breaks down, is if my shutter speed needs to go beyond 30 seconds; I would then switch to bulb mode and calculate the new shutter speed as above.
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