The Ultimate Guide To Landscape Filters: The Neutral Density Graduated Filter

Sarah Howard, from Image Seen, has put together a comprehensive guide on filters landscape photographers should have in their camera bag and in part three, ND Grads are Sarah's focus. Don't forget to check out part one and two which look at polarising and UV filters. 

Keep checking FilterZone for the next instalment which will look at ND filters. 

The Neutral Density Graduated Filter - What Is It Used For? 

Graduated filters help cameras record scenes more like we see them; with a broad tonal range, by helping us to control highlight and shadow detail.

In landscape photography they are used to balance exposure across the scene, most often the brightness of the sky with the land. When correctly used they give a much more natural looking result than HDR processing. Also, unlike applying a digital graduated filter effect, that simply places a uniform darkening tone over the areas without any detail, ND grads enable the camera to record the highlight detail and bring out the subtle tonal variations.

How Does It Work?

The camera and human eye do not see the same way. The human eye is incredibly sophisticated; our vision has a ‘dynamic range’ (or subject brightness range) of around 24 stops, allowing us to see detail in areas of both shadow and highlight, even on a bright day.

In contrast, a camera's dynamic range is far more limited. The quality and size of the sensor can determine the range but it’s typically around 8-12 stops which is very often far narrower than the dynamic range of the scene being photographed, thereby creating exposure issues.

In this instance, either the darker areas of the scene will be underexposed and appear completely black, or the brighter parts will be over exposed, indicated by the flashing areas in the image on the LCD screen. In both cases, detail is lost.

A Neutral Density Graduated filter allows the photographer to capture a scene that would be virtually impossible to photograph otherwise. By positioning the darker part of the filter so it covers the brightest section of the scene the photographer can set an exposure that retains all the detail. 

Types Of ND Graduated Filter

ND grads (for short) are pieces of glass or photographic resin that are clear in one-half, and darker in the other, with either an abrupt or a gradual shift in the middle, depending on whether you opt for a hard or soft version. Resin is lighter and more affordable than glass and also more likely to survive dropping. On the downside, they are more prone to scratches. In either case, they need to be stored carefully.

These filters come in two types; screw-on, which screw onto the front of the lens and square or rectangular ones that slip into a filter holder attached to the front of your lens. This is the more versatile option as it enables the filter to be moved up and down as well as rotated. They're also made in a range of sizes, to accommodate small, medium and large format cameras. There are several filter systems on the market, Cokin and Lee being two of the most popular. 

What Strength?

ND grad filters are available in different strengths, allowing you to choose the one most appropriate, depending on the level of contrast between the brightness of the sky and land. Most manufacturers produce a range of filters capable of cutting out one, two or three stops of light, but some, such as Lee Filters, produce grads with intermediate strengths for greater control of exposure.

It can get also confusing as different manufacturers give them different names, such as ND4 or 0.6 ND – both indicate a filter of the same density, which will reduce the exposure by two stops across the area which it covers.

Hard Or Soft?

With both types, the darkening of the filter begins in the middle. A soft filter gradually gets darker as you move from the middle towards the other end of the filter, whereas the hard type has a defined line, quickly changing from clear to dark.

Whether you use a soft or a hard grad will depend mainly on the subject matter of your image.

As a general rule, a hard grad would be used for images containing a clearly defined horizon, or any hard transition between the sky and the foreground whereas a soft grad works best where there is no definite transition between sky and foreground.

Soft grads, having a more subtle transition, are less obvious to the eye and can be trickier to position correctly because it is harder to see their effect in the viewfinder.

In addition, only a small part of the filter at the top is fully dense before it then fades out towards the centre. As the brightest part of the sky is usually just above the horizon, it becomes necessary to push the filter down further so it overlaps the ground. Whilst hard grads can be aligned with more precision, they create a more defined line between light and dark in an image. Therefore they need to be used with care as they can be spotted easily on subjects that cross from the darker part of the image into the brighter area; for example a tree or a building, resulting in an unnatural look. 

No graduated ND filter was used on this image and the sky is overexposed.

An ND Graduated filter was used here, to balance out the exposure of the sky and the land.

Using An ND Grad

The key to using an ND Grad is to select the appropriate one (strength and type) and then position it correctly so that no one can tell you’ve used one. It should not be obvious.

Positioning the filter:

Method one

1) Set the camera to Aperture Priority mode and, having selected your desired aperture, take a meter reading for the foreground being careful to exclude the sky (do this without the filter in place).

2) In the same way, take a reading for the sky and note the difference in the two readings.

3) Select the correct strength of filter to control this difference in exposure to within one to two stops (note: the sky is usually naturally brighter than the land) e.g.: if the sky is 3 stops brighter, use a 1 or 2 stop filter depending on the result you want.

4) Slide the filter into position being careful to position it so that the transition line falls exactly where required; on the point that the light and the dark areas of the scene meet. The effect should be visible through the viewfinder or on the LCD screen. (Stopping the lens down by using the depth of field preview can make this easier to see).

Method two

With digital SLR’s there is also another method, which can be used to determine which ND grad to choose:

1) Having already composed your image, take your photograph using the settings the camera calculates to be correct.

2) At this point, check your histogram and also your highlight-warning indicator. The histogram will confirm whether the highlight and shadow detail has been recorded and the highlight-warning indicator will confirm if the highlights have ‘blown’ i.e. the image is over exposed. If the histogram is towards the left-hand side or the middle of the scale, you know you need to use a weaker filter whereas too far to the right-hand side, or off the scale on the right, means a denser filter is needed.

What to watch:
1) Accurate positioning is important. If the filter is placed too high, the transition will be seen in the sky, too low, and the foreground will have an unnatural "shadow" across it.

2) Avoid using multiple filters. Aside of reducing contrast, this increases the number of reflective surfaces in front of the lens, causing possible flare.

3) Vignetting can be a problem with wide-angle lenses. This is another reason to avoid multiple filters.

4) The filter should be neutral, so it doesn't introduce any unwanted colour cast. Cheaper filters may have a noticeable colour cast (usually magenta).