The fundamental principle underlying the practice of photography is about controlling the amount of light that reaches a sensor or film. We do this by using 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 handholding at slow shutter speeds. However, there are times when there is too much light, rather than not enough. How can we deal with that? The answer is neutral density, or ND, filters. An ND filter is, in its 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, let’s 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, with exposure times running into seconds. The problem is that it is often difficult to obtain these very long shutter speeds in daylight. There is just too much light.
Image via Alejandro Lopez, Flickr
The blurred water effect is made easier with ND Filters
The other typical case where you might need an ND filter is when using flash outdoors. 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, on a beach for example, and want to use a wide aperture to obtain a shallow depth of field? It may be almost impossible to get the shutter speed down to 1/250th second, so it’s time to add an ND filter. In flash photography, the ND filter will also reduce 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 ND filter is when we need 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 at or below the diffraction limit, we can use a neutral density filter.
Image via Kaybee07, Flickr
An ND8 was used here to maintain shallow depth of field with flash
What is an ND filter?
As we said earlier, an ND filter 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 spectra by equal amounts, meaning that the image color is not compromised. There are a couple of scales that ND filters use as 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% change in light levels. The following table demonstrates the differences in shutter speed for the same aperture.
|Filter||Shutter Speed||Aperture||Light Reduction|
As you can see, the more opaque the filter, the less light reaches the sensor and hence we need to use a slower shutter speed (or a 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 either have a filter for every lens that has a different diameter thread, or to 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 those sold by Lee or Cokin. These 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’s 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.
Image via Mark Mitchell, Flickr
A Lee Square ND filter in action
A further development of the square filter range include filters such as the Big Stop, which are extreme neutral density filters, cutting down the light by up to 10 stops. The last option is a fairly new development, and that is the variable neutral density filter. These use two polarizing filters sandwiched together. By rotating the front filter, the user can control the amount of light reaching the sensor.
Image via Pulaw, Flickr
A variable density ND Filter
Neutral density filters are a very useful tool for your kit bag, especially for 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.