Neutral density filters are used to turn down the light level when the intensity is too great for you to achieve your desired photographic effect. They’re hugely important and useful in several genres of photography, though most especially in landscape photography.
There are two broad categories of these filters: a) neutral density filters (usually abbreviated to just ND filters) – that turn down the amount of light across the entire scene; and b) neutral density graduated filters (usually abbreviated to ND grads) – that turn down the light across just a part of the scene.
Starting with the ND filters, this blog will describe the use of each filter type.
What are ND filters, and why would you use one?
When shooting in the middle of the day, there will be times when there is just too much light for some kinds of creative photography to be possible.
This would be when you’d ideally want to blur some movement in the scene, whether in water, sky, trees or people, for example. You need to use a slow shutter speed for this kind of photography – anything from 0.1 seconds and upwards.
But usually, in the middle of the day, there is just too much light for you to be able to do this, no matter how much you try to close down your lens aperture or how low you set the camera sensor's ISO (i.e. its sensitivity).
Normally, you’d have to wait until dusk – or at least a very dense cloud – for light levels to be low enough to allow for such slow-shutter speed/long exposure photography, and indeed, much of the world's most creative landscape photography is done under such conditions.
But fortunately, it is possible to blur those rolling waves, flowing rivers or ambling people, even in bright sunlight, through the simple use of a neutral density filter. ND filters are darkened sheets of glass or plastic that fit onto the front of your lenses, cutting down the amount of light able to pass through the lens to the camera's sensor. They manage this without changing the colour balance in any way, absorbing all the light wavelengths equally – hence the term neutral - thus generating a final image that has the same colour balance whether shot with or without the filter.
How do you use ND filters?
Most ND filters are circular and simply thread onto the front of your lens, so you need to buy filters with the same thread size(s) as your lens(es). As a result, if you have several lenses, each with a different thread size on their front end, then you'll usually need to buy a separate ND filter for each. However, it’s also possible to buy 'step-down' rings that allow you to screw a larger diameter filter onto a smaller lens.
ND filters are available in varying strengths, ranging from a 1-stop or 1-EV change (i.e. a halving of the amount of light passing through) to about a 5-stop/5 EV change (a 32-fold reduction in the amount passing through the filter), or sometimes higher. There are also several variable filters available, in which the filter's density can be altered, a hugely convenient feature. The relative strength of an ND filter is measured on the ND scale, where ND2, for example, reduces the light passing through by 1-stop (i.e. a two-fold reduction).
When mounting any of these filters onto your lens, it’s wise to remove any other filters that may already be on the lens, such as a UV or skylight filter, as putting these together may result in vignetting (i.e. shading in the corners), especially when you're using a wide-angle lens.
Some square ND filters are also available, which mount onto the lens by sliding into a special holder that clips onto a threaded ring. These filters are used mainly for very high ND values and are often called 'stoppers' because they stop the light. They usually come with foam strips on their rear edges, which press up against the holder, ensuring that no light leaks around the edges. They’re typically available as a 'big stopper', giving a 10-stop/10 EV reduction in the amount of light passing through, or a 'small stopper', giving a more modest 6-stop/6 EV reduction. The latter represents a sizeable 64-fold reduction in light, while the big stopper delivers a whopping 1024-fold reduction!
Not surprisingly, when filters with high ND values are used, precious little can be seen through the camera's viewfinder or LCD screen. It’s generally necessary to line up, compose and focus the intended image before carefully mounting the filter. To ensure no changes while doing this, be sure to have the camera mounted on a tripod, and either have the focus locked, or the autofocus switched off once the image has been focused. Once the filter is in place, then you can fire the shutter.
Although it’s sometimes said that exposure metering should also be carried out without the filter in place, followed by some complex calculations to work out what the exposure should be once the filter is there, in my experience, this generally isn't so. The in-camera exposure meter should be able to correctly read the required exposure even through quite a dense ND filter – as far as the camera is concerned, it’s just either dusk or night time – which enables the metering to work just fine.
ND filter effects
Not surprisingly, the effects achieved vary enormously, depending on the density of the filter you use and the speed at which your subject is moving.
As a general rule, the high-density stoppers will completely blur out moving water – even large shoreline waves - for example, resulting in a smooth, milky, ethereal and a rather other-worldly scene. Any people moving around in the scene are likely to disappear completely; only those standing relatively still will remain visible.
Lower density filters (such as one with a three-stop/8-fold reduction), on the other hand are more likely to give results that mimic dusk. Waves will still be visible, though they’ll be blurred. Surfers hitting the shore will either be equally blurred or broken into what I call 'shards of glass' – i.e. sharp spears of water flying through the air. The overall effect generates an image with a lot of dynamic energy, with a sense of movement well portrayed.
Similarly, moving traffic or people will still be visible but with a blur that dynamically portrays movement and energy, mimicking what can be achieved at dusk without any filters.
Neutral density graduated filters
Neutral density graduated filters (usually shortened to ND grads) are among the most important add-ons for landscape photography and are probably more widely used than ND filters. Anyone shooting landscape photography should have a couple in their kit bag.
What are ND grads?
ND grads are a rectangular sheet of optical quality plastic, with one half darkened and the other half completely clear. There are essentially two types: hard and soft, which describe the transition from clear to dark in the filter's central area.
Not surprisingly, hard filters have a sharp transition between the two zones, whereas soft filters have a more gradual transition. Recently, some filter manufacturers have introduced medium and extra hard filter types, providing an extra bit of fine control over that all-important clear-dark transition zone.
In addition, the filters come in a range of darkening grades, usually termed 0.3, 0.6, 0.9 and 1.2. For the 0.3 filters, these equate to a one-stop/1 EV reduction (i.e. a halving) of the amount of light passing through the dark part of the filter. For the 0.6, it’s a two-stop/2 EV (four-fold) reduction; for the 0.9, it’s a three-stop/3 EV (eight-fold) reduction; and for the 1.2, a four-stop/4 EV (16-fold) reduction.
You can also buy ND grads as circular filters that screw onto the front of a lens in exactly the same way as most ND filters do. Unfortunately, these are not very useful as this puts the transition line between the dark and clear parts of the filter in a fixed place, massively restricting your choice of composition. You need to move that line to just about anywhere in the frame to maximise flexibility for a host of composition and light conditions.
From here on, only the rectangular slide-in ND grads will be referenced.
What are ND grads used for?
Everyone who has taken a landscape photograph has had the experience whereby in the final photos, either the land is well exposed, but the sky is burned-out, with all cloud details lost, or the sky looks great, but the land is lost in a dark, almost featureless zone. This can happen even though the view looks fine to the eye.
The reason for this is that although the eye can easily handle a huge contrast range between, say, a bright sky and a darker landscape, the digital sensor is just not up to that. As a result, it can correctly expose either the land or the sky – but not both.
By putting the dark part of the filter over the bright part of the view, ND grads reduce the contrast range, bringing it down to something the digital sensor can cope with, enabling it to capture all the detail in both the bright sky and the darker landscape. The result is a photo more closely resembling what you saw with your eyes. Essentially, it’s an important technical fix for a major failing in the digital sensor.
They can also be used to exaggerate the real situation. For example, increasing the apparent storminess of a cloudy sky, helping to increase the sense of drama and/or mood in the photos.
How do you use ND grads?
You fit these rectangular filters to the front of your lens, using the same holder and lens ring as for the stopper ND filters. When first kitting up, these will need to be bought in addition to the filter(s). Make sure to buy adaptor ring(s) of the right thread size(s) for your lenses.
As already mentioned for circular ND filters, when mounting the ring and holder for ND grads onto the lens, you should first remove any circular filters you may already have on your lens (e.g. a UV filter). If such filters are left in place, there is the danger of vignetting in your photos, especially if shooting with a wide-angle lens.
After sliding the ND grad into the holder, look through the camera's eyepiece or at the live screen view, and slide the filter up and down until its light/dark transition zone matches up with the landscape's horizon. Of course, this is much easier if the camera is rock steady on a tripod with the image view already composed, but you can also do it with the camera hand-held if done carefully. This process takes practice; when you're new to it, you may find it completely impossible to even see that transition line, let alone be able to line it up with the horizon. But practice makes perfect.
Once everything is lined up, you can shoot normally. The in-camera exposure meter works perfectly well with the ND grad in front of the lens.
Choosing which ND grad filter to use becomes easier with experience. Generally, a hard grad is the one to use when you have a distinct horizon (such as the sea against the sky), whereas you’d use a soft ND grad when it’s not clear where the horizon is, such as in woodland or misty/foggy weather. Choosing your grade also depends on how big the view's contrast range is. However, generally, the 0.6 grade is the most useful.
Types of cameras you can use ND grads with
Generally speaking, ND grads have been developed with DSLRs in mind, and more recently, for the rather smaller mirrorless cameras.
Although filters can come in various sizes, it’s best to use filters that are 100mm wide and 150mm long for DSLRs. This size covers use with just about any lens available on the market, including those with very wide-angle views.
Smaller filters (sometimes called A and P sizes) can be useable with smaller lenses, such as those provided for mirrorless cameras. However, on a DSLR, they can be problematic with a very wide-angle lens, often causing vignetting in the photos. Larger filters are also available for those using medium format cameras, such as Hasselblad or Phase One.
Unfortunately, it isn’t easy to use ND grads with bridge or compact cameras. Firstly, there’s usually no lens-front thread that can take an adaptor ring and holder. Secondly, the lens can quite radically change length when zooming and/or focusing, making it almost impossible to even hold the filter in front of the lens.
Equipping yourself with ND and ND grad filters
Several manufacturers produce ND and ND grad filters, principally Lee Filters, Cokin, Hoya and Hi-tech. The first of these produce what are generally considered the top-of-the-line, industry-standard filters, whereas Cokin and Hi-tech are rather more budget products. That said, all ND grad filters can be quite expensive, so if your budget is limited, there’s no need to splash out on a full set right away. Just invest in the holder, adaptor rings and one, maybe two filters initially – the 0.6 hard and 0.6 soft are probably the most useful filters in the range.
Similarly, don't go overboard when starting with ND filters. Either buy just a couple of modest fixed density filters or single variable filters initially while you learn the ropes. Once you’ve become more comfortable with ND filters, invest in one of the much more expensive stoppers.
There’s quite a significant price range with these filters, especially the ND grads. For most amateur photographers, there’s no real need to invest in the expensive top-of-the-line filters: something more middle of the road will work perfectly fine. Don't be tempted to go too cheap, however. It is possible to buy entire sets containing a mind-boggling array of ND and ND-grad filters for a very small amount of money. However, they generally don't do their job well: the large number of filters just makes for confusion, they’re often too small for most cameras (especially DSLRs), and they are prone to cause a colour cast (i.e. the darkened areas are not truly neutral).
So, once you've got yourself a basic set, just get stuck in, get creative, and enjoy this newfound form of beautiful photography!
This blog was written by Nigel Hicks, a hugely experienced Devon-based professional photographer. Nigel works with the USA's prestigious National Geographic Image Collection, among many other bodies, and is a Fellow of the British Institute of Professional Photography.
Nigel runs regular photography workshops in southwest England. To find out more about these, go to https://www.nigelhicks.com/photography-workshops-courses/.
To find out more about Nigel's work in general, feel free to take a tour of his website at www.nigelhicks.com.
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