In the film industry there is a term called "Motion Blur", it's used to describe the Blur of moving objects within a single frame or a series of frames. Motion Blur can also be caused by the movement of the camera itself, the cameras movement blurs the images in the same direction in which ithe camera is being moving. When shooting with a 35 mm motion picture camera or more often these days a high resolution digital movie camera the frame rate is 24 frames per second with an exposure length of 1/48th of a second per frame. Frame rates do vary and so do exposure lengths depending upon the desired wishes of the Director and the Director of Photography. However 1/48th of a second is extremely common. Now 1/48th of a second is quite long when compared to shooting with a digital still camera on a bright sunny day where the exposure length might be 1/2000th of a second. The difference between these two exposure lenghts is massive, the frame shot at 1/48 will have a slight blur on any objects which were moving when the frame was exposed. The digital still camera which had a 1/2000th of a second exposure will be sharp as a tack even on the objects which were moving (unless they are moving extremly fast).

It can be argued that having motion blur is a good thing when it comes to time-laspe. You get a sense of movement in your images. Motion Blur also helps to cut down on what can be perceived as a strobing effect on moving objects when shot at a high shutter speed. Adding motion blur to a time-lapse sequence of moving clouds helps to give a better sense of movement. Anything which is blurred in the direction of it's movement conveys movement. Personally I love introducing this creative element into my time-lapse sequences where possible. Below are two still frame examples both shot seconds apart on a sunny day, one has motion blur, the other does not.

bvf_sharp bvf_blur
No Motion Blur
1/800th of a second exposure
With Motion Blur
1/5th of a second exposure

Ok, so how is this done? What extra equimpment is required and what camera settings are best suited to acheive motion blur? As far as extra equipment goes a Netural Density filter or two is about all you need. This type of filter is also known as an ND Filter, it's only job is to reduce the amount of light entering the lens and ulitmitly hitting the sensor. ND Filters come in many different values, the darker the filter the less light enters the camera. Or to put it another way, the darker the filter the longer the shutter will have to stay open to obtain a proper exposure.

It's not always nessesary to use an ND filterto obtain motion blur. If you are shooting in low light perhaps at dusk, inside an enclosed space with limited light or at night then you can acheive this effect just by ensuring that the camera's shutter is slow enough to capture the movement of the objects in your frame.

Camera settings play a huge role in determining how much or how little motion blur you want. This is done by varing the length of the shutter, adjusting the apeture and in some cases changing the ISO number. Basically it comes down to this. The longer you want the blur of the moving objects in your frame the longer the shutter will have to remain open. Don't forget, the speed at which the objects are moving also plays a very important part of the equasion. For example the motion blur from traffic moving along the freeway at 120 kms per hour vs someone walking along the street will be drasticlly different when you use the same exposure length.

So grab yourself a few ND filters which will fit your largest lens. That way you can always use the same filter on smaller lens with the addition of a Stepdown Filter Ring (See below). Search Google for more information on Stepdown Rings.

stepdown NDX400 In my photography toolkit I have a 9 stop ND Filter (NDX400) made by HOYA, its like looking through welding glass. This filter cuts the amount of light entering the lens down by 9 full stops. Or in simpler terms, it lets in only 1/512th of the light. This will allow you to have very long shutter lengths even on a bright sunny day. Here is a great website to purchase this filter or many others as well, their customer service is extremely good,
Stepdown Filter Ring HOYA 9 Stop ND Filter