The art of photography is all about the balancing of light. In a camera we have two basic tools that help us to control the amount of light that reaches the sensor, the aperture and the shutter. In most cases these two controls are enough to balance the light and give us the correct exposure. In times when there is not enough light, we have a third tool at our disposal, ISO. Raising the ISO, increases the sensitivity of the sensor, allowing us to shot in much lower light levels than available at the base ISO. However, there are times when we need to reduce the amount of light reaching the sensor whilst maintaining a wide aperture or slower shutter speed. Most cameras do not have a built in way to do this, and so we need to use neutral density filters. A neutral density filter is usually a piece of darkened glass that is placed over the lens in order to reduce the amount of light reaching the sensor or film. Before we go into details about neutral density filters, lets take a look at some typical reasons as to why you might need to use one.
Using Flash Outdoors: Most DSLR cameras have a maximum flash sync speed. This is the fastest shutter speed at which a flashgun will produce a correct image when connected to a camera. Typically this is around 1/250 of a second. Now if, for example you want to take an outdoor portrait on a beach in bright light and want to use a wider aperture for a shallow depth of field, you might find there is too much light and you cannot get the shutter speed low enough to sync with the flash. By using an ND filter you may be able to reduce the shutter speed enough to sync. Of course by blocking light to the sensor, you will have to increase the amount of light that is output from the flash to compensate.
Using an ND to obtain flash sync by Severin Sadina on Flickr
Motion Blurred Seascapes: One of the most common uses of neutral density filters is in landscapes and seascapes. The ethereal looking water we often see in beach, sea and waterfall shots is created by using an ultra slow shutter speed. Whilst it is possible to reduce the camera’s aperture to its minimum, this is often not enough to get the very slow shutter speed required, and so we use an ND filter to reduce the light even further.
Blurred Seascape using an ND Filter by NoahSud on Flickr
Keeping Within the Diffraction Limit: All camera/lens combinations have what is know as a diffraction limit. What this means is that when you go beyond a certain aperture, typically f8 on APS-C sensors and f11 on full frame sensors, the image starts to suffer from a loss of sharpness caused by diffraction. To counter this, we can use ND filters to bring our aperture back below the diffraction limit.
What is a Neutral Density Filter?
An ND filter is a piece of translucent acrylic or glass that sits in front of the lens and reduces the amount of light reaching the sensor or film by a set amount. Typically the amount of light reduction achieved by an ND filter is defined in stops, one stop being a 50% reduction in the amount of light. However the naming convention for ND filters can be as a decimal i.e,. 0.3, 0.6 or as an ND number ND2, ND4 etc. The table below demonstrates to effect of different filters on the shutter speed of a typical exposure.
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 see, the more opaque the ND filter the slower the shutter speed that is required to create the same image. The same is true for aperture, if we keep the shutter speed fixed, the aperture would be reduced from f8 to f5.6, f4 and f2.8 respectively.
Types of ND Filter
There are basically two ways that an ND filter can be fitted to your camera’ lens and two materials that it can be made from.
Screw Filters: With a screw filter, the ND filter is encased in a circular holder that screws directly onto the filter thread of your camera. The main disadvantage of this is that if you have lenses with different sized filter threads, you will need several different filters or filter step down adapters. On the pro side, these filters tend to be cheaper.
Square Filters: Square filter systems use an attachment that sits on the front of the lens allowing one or more typically 75mm or 100mm squared filters to drop into the attachment. The primary advantage to this is that you can have attachments for each lens and only one set of filters.
Acrylic Filters. These are most typically for the square filter system, however because of their plastic nature, they are prone to scratching and the image quality is not as good as with glass.
Glass Filters: The best quality ND filters are made from glass. They can be either in the square format or as screw in filters. They tend to be heavier and more expensive than their acrylic counter parts but will yield a much better image quality.
Specialist Neutral Density Filters
In recent years there have been a number of specialized ND filters that have come onto the market. The two most common are the variable ND filters and the extreme ND filters such as the Lee Big Stopper.
A Seascape using an extreme ND by Alfredo S. on Flickr
Variable Neutral Density Filter
The main disadvantage to neutral density filters is that to be entirely flexible in your shooting you need to carry a range of different NDs. This can become an expensive proposition, especially if using screw filters with different lens filter sizes. To counter this some manufacturers have created variable ND filters. These work by effectively sandwiching two polarizing filters together. The rear polarizer will cut out light in one plane, whilst the from element can be rotated, effectively reducing the light in the other plane. By using this technique you can control the amount of light reaching the sensor with almost infinite control.
The advantages to this are that you get multiple ND filters in one package, the disadvantage is a loss of image quality caused by both using two elements together and by using two polarizers combined.
Extreme ND Filters
To create ethereal looking landscapes and seascapes with extremely blurred water or other motion has needed the use of multiple stacked ND filters. This had, as in the case of variable NDs, the effect of reducing image quality. To counter this some manufacturers have produced high quality, extreme ND filters. Typically these are rated at a 10 stop reduction, allowing for very slow shutter speeds even in relatively bright conditions.