The very basics of photography is about controlling the amount of light that reaches a sensor or film. We do this with three primary controls, aperture, shutter speed and to a slightly lesser extent ISO or film speed. Many of us are aware that when the light levels are too low and we do not have a tripod we can increase the ISO of our sensor to counter the potential camera shake we might incur from hand holding at low speed, however, there are times when there is just too much light rather than not enough, how can we deal with that? The answer is neutral density or ND filters. ND filters are in their simplest form a piece of darkened glass that sits over your lens and reduces the amount of light reaching the sensor.
Why do we need ND Filters?
Before we look at ND filters, lets take a look at some scenarios where you might need to use one. The most obvious usage is in landscape and seascape photography where you get that ethereal looking blurred water, caused by its motion. To get this effect you need to have a very slow shutter speed, running in to seconds. The problem is, even in the very lowest available light, it is often difficult to obtain these very long shutter speeds, the there is just too much light.Â
The other classic case where you might need an ND filter is using flash out doors. As many of you will know, most DSLRs sync flash at around 1/250th of a second, but what if you are shooting a model outside, for example on a beach, and want to use a wide aperture to obtain a shallow depth of field. It may be almost impossible to get the the shutter speed down to 1/250th second so it’s time to add a neutral density filter. Of course in outdoor flash photography, the ND filter is also stopping the flash light reaching the sensor so you will need to have a powerful flash gun to overcome this.
The last, common scenario where we might use an neutral density is to keep our aperture within the diffraction limits. Diffraction is difficult to explain easily but in layman's terms when the aperture of a lens is smaller than f8 on APS-C sensors and f11 on full frame, there is a small but significant softening of the image, giving us a perceptible loss of quality. In order to keep our exposure on or below the diffraction limit we can use a neutral density.
What is an ND Filter?
As we said at earlier a neutral density is a piece of glass or sometimes acrylic that sits in front of our lens and reduces light coming to the sensor. The term neutral comes from the fact that the filter reduces light in red, green and blue spectrums by equal amounts, meaning that the image color is not compromised. There are a couple of scales that ND filters use as a naming conventions, these are 0.3, 0.6, 0.9… and ND2, ND4, ND8….. The difference between each filter is exactly one stop or a 50% reduction in light. The following table demonstrates the differences in shutter speed for the same aperture
Filter |
Shutter Speed |
Aperture |
Light Reduction |
None |
1/500 |
f8 |
None |
0.3/ND2 |
1/250 |
f8 |
1 Stop |
0.6/ND4 |
1/125 |
f8 |
2 Stops |
0.9/ND8 |
1/60 |
f8 |
3 Stops |
1.2/ND15 |
1/30 |
f8 |
4 Stops |
As you can clearly see, the more opaque the filter the less light reaches the sensor and hence we need to use a slower shutter speed, or wider aperture.
Types of ND Filters
Lastly let's look at the types of NDs that you can buy. The cheapest option is the round, screw in type filter that matches the filter thread on your lens. The biggest problem with this is that you will need to have either a filter for every lens that has a different diameter thread or carry a set of step down rings to convert your filter to the right thread.
The second option, is the square type filters such as Lee or Nisi. This are generally more expensive but by using a simple filter holder on the lens you can quickly change filters or stack multiple filters together. The more expensive filters such as Lee are made from high quality optical glass whilst cheaper options like the Cokin range are generally made from acrylic which can scratch and reduce image quality.Â
A further development of the square filter range are filters such as the Big Stop, which are extreme neutral density filters, cutting down the light by 10-15 stops.
The last option is a fairly new development, and that is the variable neutral density filters. These use two polarising filters sandwiched together. By rotating the front filter the user can infinitely control the amount of light reaching the sensor. These are ideally suited to video work where it's important to keep your shutter down to the 180 degree rule.Â
Neutral density filters are a very useful tool for your kit bag, especially those landscape and outdoor portrait photographers amongst you. As with all things there are prices to suit everyone but if you look at them as an investment in your photography, go with the better quality square system with glass filters. Looked after well, these will last for many years of intense photographic use and will prove to be an excellent purchase.