Tutorial Guide - Time Lapse Photography with a Neutral Density Filter

The process of creating a moving of an image from a series of photos (time lapse photography) can have the undesirable effect of jittering when objects move quickly in the scene. The jitter can be smoothed and blurred using a neutral density filter. The video below shows an example of this jittery effect followed by footage of the same scene using a neutral density filter.

A neutral density filter is designed to reduce the amount of light entering the camera lens evenly over the colour spectrum (hence a neutral filter). The reduced light allows you to significantly increase the exposure time and capture objects that are moving quickly through the scene resulting in a motion blur rather than an instant snap shot.

Neutral density filters are ideal for daytime or well lit time lapse scenes containing rapidly moving elements such as

• Crowds of people, such as a conference or market place
• Waves on the water
• Trees swaying in the breeze
• Wind mills or propellers
• Traffic

The one drawback of using a neutral density filter is the low light operating conditions of a digital camera, you will need to consider the following.

• if the exposure time is greater than 1 second, your camera may need to take a dark exposure to compensate for CCD imperfections. For example if you set an exposure time of 10 seconds, the camera may take 20 seconds to complete the exposure (plus overhead for recording the image to the memory card)
• the increased exposure time will significantly reduce the battery life of the camera, this is worth considering particularly since a time lapse film may potentially consist of thousands of photos.
• images may become grainy if the exposure time is very long.

With these limitation in mind you will need to choose an appropriate exposure time, in my experience a suitable exposure time would be 1/4 to just under 1/2 of the interval time (time between photos). In the video above the exposure time was 2 seconds, with an interval time of 5 seconds.

Type of Neutral Density Filters

Neutral density filters come in a range of strengths or attenuation factors, typically these filters are labeled as ND2 or ND1000, where the number after the ND is the attenuation factor.

You can determine how much light is passed through the filter by dividing 1 by the attenuation factor, for example an ND2 filter has an attenuation factor of 2, and the amount of like which passes through the filter is 1/2 which is 50%. An ND8 filter lets only an eighth (1/8) of the light pass through or 12.5%

When choosing a neutral density filter it is worth while visiting your local camera store to seek advice on which filters best suit your digital camera. Some cameras require a special hood adapter to allow you to attach a filter. Filter also come in a range of sizes and it will be nessesary to use a filter of the correct size, or to purchase stepping rings that will allow you to use a filter of a different size on your particular camera.