Using Neutral Density (ND) and Graduated Neutral Density Filters (GND)
The purpose of standard photographic neutral density filters is to allow the photographer greater flexibility to change the aperture, exposure time and/or blur of subject in different situations.
Examples of this use include:
- Blurring water motion (e.g. waterfalls, rivers, oceans).
- Reducing depth of field in very bright light (i.e. daylight).
- When using a flash on a camera with a focal-plane shutter exposure time is limited to the maximum speed -often 1/250th of a second, at best- at which the entire film or sensor is exposed to light at one instant. Without an ND filter this can result in the need to use f8 or higher.
- Using a wider aperture to stay below the diffraction limit.
- Reduce the visibility of moving objects
- Add motion blur to subjects
For example, on a very bright day, one might wish to photograph a waterfall at a slow shutter speed to create a deliberate motion blur effect. In order to do this, one would need a shutter speed on the order of tenths of a second. There might be so much light that even at minimum film speed and a minimum aperture such as f/32, the corresponding shutter speed would still be too fast. In this situation, by applying an appropriate neutral density filter one or more stops can be taken out of the exposure, allowing a slow shutter speed and more pleasing effect.
ND Filter identification
ND filters come in different strengths. There are two ways used by different manufacturers of indicating those strengths:
ND filters are quantified by their optical density or equivalently their f-Stop reduction as follows:
| Attenuation Factor | Filter Optical Density | f-Stop Reduction | % transmittance | ||
| ND2 | 2 | 0.3 | 1 | 50% | |
| ND4 | 4 | 0.6 | 2 | 25% | |
| ND8 | 8 | 0.9 | 3 | 12.5% | |
| ND16 | 16 | 1.2 | 4 | 6.25% | |
| ND1000 | 1000 | 3.0 | 10 | 0.1% |
What’s A Graduated Filter?
It’s all in the name. To put it simply one half of such a filter (grad) is darker than the other, while the other half – in most cases – is completely clear. The filters job is to reduce the brightness difference between the sky and the ground, thereby allowing your digital sensor to record detail in both these areas. A neutral density grad shouldn’t cause any color casts on the sky, some cheaper grey grads can often produce unwanted colors.
Neutral density grads are given numbers which tell you exactly how many stops of light they’re going to reduce the brightness by, while with grey grads it’s much more hit or miss. So if you can afford it, neutral density grads are the ones to go for, although a grey grad makes a good alternative.
Positioning a Graduated Filter
All graduated filters need careful positioning to get the best effect. To make the most of this your camera should be fixed securely to a tripod – which is always a good idea for landscape shots anyway. This allows you to slide the filter accurately into position, so the transition from clear to dark falls on the horizon. If your camera has a depth-of-field preview facility that stops the lens down while you’re looking through the viewfinder then use it. The darker viewfinder image will make it easier to see the position of the filter.
Determining Exposure with a Graduated Filter
The whole reason for using a neutral density graduated filter is to control the exposure difference between the sky and the ground. You’ll need to take control of the light metering to make full use of the effect. Ideally you’ll set your camera to manual so that the settings don’t change when you start using the filter.
Your first step is to determine the strength of filter you’ll need. The simplest method is to take a meter reading with the ground filling the whole of the frame without the filter in place. Now repeat this step but this time with the sky filling the frame. The difference between these two readings will indicate the strength of graduated filter that you need. A 1-stop difference in the readings will need a 0.3 ND graduated filter, a 2-stop difference a 0.6 ND grad, while a 3-stop difference will require a 0.9 ND grad. If you’re only going to buy one graduated neutral density filter, the 0.6 ND is the one to buy. Once you’ve decided on the filter you need, use the meter reading you took from the foreground to take your shot.
