Every photo, or the individual frames of a video, are just samples in the continuous flow of time. Whether it’s an electronic shutter or a mechanical one, video or stills camera, the sensor is exposed for a pre-determined time to the image formed by the lens.
If this image moves during the exposure, each part of the image will leave its mark in the form of a smear rather than a single point. This happens even when the image is perfectly focused.
This relative movement between the camera and the subject during the exposure is what causes motion blur.
Motion Blur vs. Camera Shake
If the subject is not moving but the camera is, this blur is usually called camera shake.
You can use it to make a stationary object appear to be moving if you move the camera in a controlled manner. More often than not, however, it’s a result of holding the camera in your hand rather than mounting it on a tripod.
Modern DSLR cameras go to some lengths to mitigate the effects of unwanted camera shake. They use various image stabilisation systems either in the camera body or the attached lens.
Image stabilisation can be very effective. But it only works to reduce motion blur resulting from camera shake. It can’t help with motion blur caused by the subject moving during the exposure.
Fortunately, this is precisely the type of motion blur we want in order to imply movement in both still images and time lapse video.
What Leads to Motion Blur
Most of the images you’ll take for time lapse material will be with the camera mounted on a tripod. Remember to turn off any image stabilisation first. Otherwise the system can work against you and actually introduce unwanted jitter.
The amount of motion blur you’ll see on a photo depends only on two factors:
how fast the image is moving on the sensor;
how long the sensor is exposed for.
The speed at which the image moves across the sensor depends upon the speed of the subject (resolved in 2D space to the focal plane of the camera) and the angle of view of the lens.
Some subjects simply look more natural with certain shutter speeds. You can freeze waterfalls to show the individual drops of water.
Most people, however, agree that slow shutter speeds work best to evoke the fluid motion. If over-done, however, it can be something of a cliché.
A slow shutter speed of 1/4 second is more than enough to blur the water movement and imply fluidity. Longer than this will turn the waterfall into a milky mist.
If you don’t allow any motion blur in subjects that we all know should be moving, the result can look unnatural.
At a shutter speed of 1/4000 sec, the helicopter’s rotors look strangely static.
Conveying the idea of movement in photography is both a technical and an artistic endeavour. Too little blur and the subject will appear static and lifeless – too much and it will disappear into a mist.
One way to convey movement in a single photo while using a fast exposure, is to combine several instances in time into a single photo.
Let’s look at this example taken using a flash in burst mode.
A single exposure to capture motion using a flash working in burst mode. Each flash freezes motion to give the impression of movement in one shot without blurring.
Here, there is no blurring as the flash duration is very fast (less than 1 ms). We could, in principle, string together a series of images each taken at a very fast shutter speed to achieve the effect of motion in a video without blurring.
If we were shooting a 4K video, each frame would be nice and clear. We could use any frame as a stand-alone photo in its own right. This sounds like a good idea, but is often not the best option.
Selecting the Right Shutter Speed for Time Lapse Video
When taking a single photo, provided you can achieve the correct exposure, you have the option of using a very wide range of shutter speeds from typically 1/4000th second to several minutes.
However, once you start shooting images for a time lapse sequence, your options become slightly more limited. The longest exposure you can use will be limited by the time interval between each photo.
For example, if you wanted to shoot one image every four seconds, you could still shoot at fast shutter speeds if you have enough light. Your longest exposure would have to be a little short of four seconds to allow the camera to prepare itself for the next shot.
This is still a huge range, so how do you choose the right shutter speed?
For some situations such as making a time lapse of the night sky, the limited amount of available light will mandate a long exposure time of maybe twenty seconds or so. This will, in turn, limit the rate at which you can take each photo.
However, when taking photos during the day with plenty of light, the decision is not so simple. The exposure time you select will have a significant impact on the mood of your time lapse. To illustrate this I took two time lapse videos – one using a very short exposure and one using a long exposure.
Fast Shutter Speed Example
In this example, I’ve made a short time lapse of some road works. To make this five second clip, I needed 150 photos for a 30 fps video.
I wanted to grab ten minutes of real time. The interval needed to be 600 seconds divided by 150 giving one photo every four seconds.
I deliberately selected a shutter speed of 1/500 second. This freezes the motion of every vehicle in the scene, from motorists to construction vehicles.
There is some sense that the construction vehicles are moving smoothly but the normal traffic movement looks random and manic.
If the shooting interval were much longer, no single car would appear in two successive frames. Assorted vehicles would appear to pop in and out of existence like quantum particles.
It’s useful to think of the exposure time as a fraction of the shooting interval expressed as a percentage. In this case, it’s 1/500/4 which is 1/2000 or only 1/20th of one percent.
Things start to look very different if we bump this proportion up to say 25 percent.
Slow Shutter Speed Example
In this example, I’ve taken another 150 photos at the same interval but set the shutter speed to one second which is 25 percent of the shooting interval.
This is a much greater proportion than the previous example.
The resulting blur conveys the flow of the passing traffic much better.
The 180 Degree Rule
As a general guideline, an exposure of somewhere around 50 percent of the shooting interval produces a natural looking motion blur. For the last example, that would have required an exposure time of two seconds.
We might, as photographers, call this the “Fifty Percent Rule”. The concept comes from cinematography and dates back to the days of rotating mechanical shutters used on film cine cameras.
These cameras used a rotating shutter that exposed the film for a proportion of a complete revolution and then hid it while the next frame was being positioned for the following exposure.
It was thus convenient to talk about how many degrees of rotation of the shutter were given over to exposing the film. Here’s an illustration showing the timing as it relates to my second example of a one-second exposure (90 degrees) and a two-second exposure (180 degrees).
The 180 degree rule is old-school cine parlance for a 50% duty cycle at which the motion blur looks natural.
Like so many photographic ‘rules’, take this one as more of a guideline. It will give you a good idea of the sort of exposure you’ll need to obtain the same amount of motion blur commonly used in cinematography.
Time lapse video produced from a sequence of stills, as in theses examples, is different from that captured by a video/cine camera. The sample rate for cine/video is typically 24, 25 or 30 frames per second.
Applying the 180 degree rule would give typical exposure times of around 1/50 second. On the other hand, making a time lapse sequence from stills shot several seconds apart, often requires much slower shutter speeds.
In the first example clip, I’d set my camera to a shutter speed of 1/500 sec at f/8 ISO 200. In order to get down to a shutter speed of one second, I might have tried to reduce the ISO to 100 and then close the lens right down to f/22.
That, however, would only give me a shutter speed of 1/30 second. Still far too fast (only three degrees in terms of spinning shutters) to record an obvious blur.
Neutral Density Filters
The solution is to cut down the amount of light entering the camera in much the same way as donning a pair of sunglasses. The important consideration here is that the light must not be polarised, tinted or otherwise filtered.
It must just be attenuated by a known amount evenly across the whole scene. This is the job of the Neutral Density (ND) filter.
In my second example, the slowest shutter speed I could get was 1/30 second. I needed to reduce the light by a further five stops to achieve a shutter speed of a full second. You can easily do this by attaching the correct ND filter to your lens.
You might imagine that ND filters are marked simply by the number of stops of light attenuation they provide but that’s not the case.
There are, in fact, two competing ways to label ND filters as shown by this table:
Filters marked ‘ND’ followed by a number eg ND4 are telling you the fraction of light transmitted (in this case 1/4). The other designation in use is a number followed by ND. This is based on the logarithm of the ratio of incident to transmitted light.
I don’t like either system and just mark my ND filters by how many stops of attenuation they provide. I only find it necessary to carry two strengths – a 3-stop and a 10-stop.
You can use a 3-stop ND very easily because it’s still possible to see through it to compose your shot. A 10-stop, however, is nearly opaque.
In my second example, I needed an extra five stops to get down to a shutter speed of one second. But the ND filter I had with me was a 10-stop filter (ND1000 sometimes known as the ‘big stopper’).
Working backwards from a one second exposure (1, 1/2. 1/4, 1/8, 1/16, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000) for ten stops gets me to 1/1000 sec shutter speed. With the camera set to manual mode, I set this shutter speed and dialled in the correct aperture and ISO to achieve a good exposure.
After checking my focus and framing, I simply set the shutter speed to one second, attached the ND filter and started my intervalometer.
Variable ND filters
Good quality ND filters are expensive and you may want two or three strengths. It can be tempting to consider the so-called variable ND filter.
These comprise two polarising filters you can rotate with respect to one-another to attenuate light. The angle between the filters determines the amount.
Looking at the markings on this image of a Vario variable ND filter, you might think that you could rotate the ring to increase the opacity.
In fact, the opacity will change slowly at first and then very rapidly as the two polarisers approach an angle of ninety degrees to one-another.
This is Malus’s Law. In practise, it makes these filters very difficult to set to a known value.
This is Malus’s law and it makes this type of filter difficult to use.
A neutral density filter should not affect any property of the light except its intensity. But this type of variable filter will select certain planes of polarisation thereby altering the image in unwanted ways.
They also tend to introduce uneven attenuation and false colour when approaching full cross-polarisation. If you dial in the maximum setting for a really slow shutter speed you could end up with something like this.
A 30 second exposure achieved using a variable ND filter showing a band of false colour.
Motion blur in time lapse photography is a useful technique to master, made that much easier by good quality (non-variable) ND filters.
Use the 180-degree rule as a guideline and enjoy the unique creative possibilities of combining slow shutter speed and normal frame rates that are not possible with normal video.
A note from Josh, ExpertPhotography's Photographer-In-Chief:
Thank you for reading...
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