Neutral Density Filters

This article aims to be a relatively simple guide to neutral density filters. The neutral density (ND) filter is one of those tools that should be in almost every photographer’s camera bag. However, ND filters are mysterious to some, and many folks just don’t understand how, where, and when to use them. Beyond that, manufacturers seem to vary in their preferences as for as how they name ND filters—adding to the confusion. In this article, I am going to try and explain the world of the ND filter; see if we can make sense of the numbers and also explore some appropriate times for their use.

What is a Neutral Density Filter?

The ND filter, (and this includes the variable ND filter), is a dark filter which is set up in front of your lens. It either, screws directly into the lens, or if it is part of a filter kit such as the 10 stop ND filter kit which I use, it slots into a holder which in turn is clipped onto an adapter which is screwed onto the lens barrel. The main difference is that I don’t have to unscrew the filter from the lens, I can remove the filter from in front of the lens by simply releasing the holder. 

Where can you use a10 Stop ND filter to great effect?

Basically in any photograph where you want to emphasise movement through blur. Popular subjects include waterfalls, moving vehicles, sport, people (although with people it is normally the background which is affected), seascapes, rivers, streams, clouds, and smoke. To be honest, your imagination is the only barrier.

The ND filter reduces the amount of light hitting the sensor. ND filters come in a variety of densities, from 2 all the way up to 24! It takes a bit of practice to understand which filter to use for different light conditions. I use the 10 stop ND filter from K&F, it is very reasonably priced with great performance. All of the images on this page were taken using this filter.

The Weir, River Severn, Shrewsbury

This image of the weir on the River Severn at Shrewsbury was captured mid afternoon on a very bright day. My objective here was to capture the way the reflections were bent by the moving water at the top of the weir, but still retain the feeling of movement and power in the weir itself. I was using an ND1000 on auto ISO. After a few practice shots I decide that f/11 @ 10seconds was the sweet spot to get the effect I was after.

Understanding ND Filters?

Depending upon the effect I am trying to achieve or the prevailing conditions, I will use one of several types of ND filter. If you aren’t aware there are different grades of ND filter, from ND2 through ND24, plus hard and soft edged graduated filters.

A solid ND filter has the same transparency for its whole surface, whereas Graduated ND filters have different blends of transparency. What do I mean by that? The easiest way to explain is by showing you. Look at the image below.

If we look at this image from left to right, the leftmost image has a soft graduation, (the change from clear to dark is spread out making the delineation very soft and gradual), whereas the image next to it has a hard or well defined graduation, (the change from clear to dark is delineated very sharply.) The two images to the right show the medium versions of these graduations.

The outer two images show a variation of the soft graduation while the centre two images show a variation of the hard graduation.

Hard vs Soft Graduated Neutral Density Filters

The choice of using a hard grad as opposed to a soft grad, is dependent upon the scene you are shooting.

When to Use a Hard Edge Graduated ND filter

In a scene where the horizon is well delineated usually calls for a hard edge ND filter. A hard edge ND filter has an abrupt line where dark transitions to clear. They are best used for scenes where the horizon is flat without any obstructions, such as with seascapes or when shooting straight out to the horizon at sea.

When to Use a Soft Edge Graduated ND Filter

A soft edge graduated ND filter features a very gradual transition from dark to clear. The top of the filter will be the darkest, gradually changing so that the middle of the filter is light while the bottom half is generally clear.

They are a good choice when there are obstacles in the landscape that rise above or obstruct the horizon line, such as mountains and trees. The softness of the gradual transition should allow for these objects to be exposed correctly when the graduated filter is aligned on or near the horizon line. The image below is a perfect example.

Using an ND filter allows you to use a wider aperture, and therefore a shallower depth of field in brightly lit conditions, without overexposing or blowing the highlights.

However, the majority of photographers use an ND filter to extend the shutter speed, or exposure time.

By adding an ND filter to the mix, you effectively shut down the amount of light entering the camera. The objective here is to blur any moving elements. The amount of blur is now dependent upon your chosen exposure time.

Waterfall, Cardingmill Valley

In this image I simply altered the exposure time, (shutter speed), to get the two different effects in the water. The left had a shorter exposure and as a result the water has far more definition than the image to the right.  As with all images your preference is subjective.

Blurring the moving elements in a scene can create some beautiful effects, to the point where a simple image can become fine art in its appearance.

Ballet Dancer

In this image I used an ND2 with an exposure time of 2 seconds.

What do the numbers on ND filters mean?

ND filters come in different transparencies. They range from ND2 to ND24, plus they can be stacked to extend exposure times.

Unfortunately these numbers do not help when trying to figure out how many stops a particular filter will apply. There are charts like the one below: –

and there are apps such as Time expert and Lee’s big stopper.

Personally I do not use either. I find it much easier to work with the camera in Manual mode, on auto ISO, with the live view in operation. Playing with the aperture and f number, gives me a fairly good representation of the exposure I will get in the final image.

River & Bridge, Leominster

Believe it or not it was very gloomy. It was early evening and the overhanging trees were restricting the light quite badly.

This was captured in the early evening. The sky was featureless and there was no wind. The ND filter has done a great job of smoothing the surface of the water just before it hits the rocks, and has even smoothed out the ripples caused by the obstruction to the flow. It was very gloomy under the trees, hence the wide aperture and long exposure time needed to get a perfect exposure.

Using Neutral Density Filters

Stacking Filters

A lot of photographers use ‘filter stacking’, effectively turning a ND2 plus an ND6 into an ND8. 

The downside to stacking filters is that, for each filter you add, you are forcing light to pass through multiple elements. The more physical layers that the light has to penetrate before hitting the sensor, the more it is likely to get slightly refracted in some way that causes softness or chromatic aberrations in an image.

Filter Kits vs Single Filters

Filter Types

A single screw in ND filter will only fit a lens with the same thread size, but if like me you have a variety of lenses then that’s a problem. For example, a single 87mm screw on ND filter is great if you only intend on using one particular lens with an 87mm thread. However, if you want to use it with a variety of lenses with a variety of thread sizes, then you need a filter kit.

I use the K&F ND1000 filter kit, which I think represents brilliant value and fab performance. The kit comes with a range of lens adapters which screw directly into the lens itself. The filter holder then clips to this adapter, and can be rotated to accommodate your graduated ND filters. The adapter features two slots. You can stack ND filters or add a polarising filter, (especially useful for water.)

One huge benefit of the kit over a single screw in filter is the ability to remove the filter very quickly if the need arises to take a sample shot or to refocus.

Other Types of ND Filters

Variable Neutral Density Filter (VND)

The VND filter lets you change the amount of density, the amount of filtration, by turning the outer ring of a dual-ring filter. I have a 2-4 stop variable ND which I find particularly useful for my wedding photography. Normally, when I am faced with bright midday sun, as I frequently am, I would have to shut the aperture down to preserve the highlights, but using the variable ND filter allows me to remain wide open and not burn out the highlights.

The disadvantage of the variable ND filter is that, as you approach the upper limit of the ND2 and the lower limit of the ND4, you get a weird green banding. However, this is easily overcome by slightly dialing it away from the outer limits.

Recommended ND Filters

If you ask landscape photographers they recommend that you head out into the field with a 6-stop ND filter which is perfect for slowing your shutter speeds enough to show smooth motion in mountain streams and waterfalls. Coupled with a Polarising filter which has a density of 2, you then have 8 stops at your disposal.

As I mentioned above I use a 2-4 variable ND for my weddings but a lot of wedding and portrait photographers prefer the 3-stop ND filter to give them a wide-open aperture option while shooting in bright conditions.

I use the 10-stop ND filter for creating some great effects where subject matter has any moving parts. I am moving towards creating fine art style images with ultra long exposures which require as much as 16 to 24 ND stops.