Motion blur has been a popular effect in photography for a long time. But you can use motion blur for time-lapse photography as well.
This article explains how.
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Motion Blur vs. Camera Shake
If the subject is not moving but the camera is, this is camera shake.
You can use it to make a stationary object appear to be moving. You just need to move the camera in a controlled manner. It’s often a result of holding the camera rather than using a tripod.
Modern DSLR cameras can reduce the effects of unwanted camera shake. They use image stabilisation systems in the camera body or 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.
Luckily, this is the type of motion blur we want. It manages to imply movement in still images and time-lapse video.
How Is Motion Blur Created?
You will use a tripod for most of the images you take for time-lapse. Remember to turn off any image stabilisation first. Otherwise, the system can work against you and introduce unwanted jitter.
The amount of motion blur you’ll see on a photo depends 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 on two things. The speed of the subject and the angle of view of the lens.
So, a fast-moving subject, narrow field of view of a long focal length lens, and long exposure times all work to produce more motion blur.
What Is Motion Blur in Photography?
Some subjects look more natural with certain shutter speeds. You can freeze waterfalls to show the individual drops of water.
Most people agree that slow shutter speeds work best to evoke the fluid motion. But f over-done, it can be something of a cliché.
If you don’t allow any motion blur in subjects that should be moving, the result can look unnatural.
How to Photograph Motion
Conveying movement in photography is both a technical and 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 is to combine several instances in time into a single photo.
Let’s look at this example taken using a flash in burst mode.
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 rapid shutter speed. With this we can achieve the effect of motion in a video without blur.
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.
What Is Right Shutter Speed for Time-Lapse Video?
When taking a single photo, you have the option of using a very wide range of shutter speeds. (Provided you can achieve the correct exposure.)
These typically range from 1/4000th second to several minutes.
However, once you start shooting images for a time-lapse sequence, your options become more limited. The time interval between each photo will limit the longest possible exposure.
Let’s say you wanted to shoot one image every four seconds. You could still do this at fast shutter speeds if you have enough light. Note that your longest exposure would have to be a little short of four seconds. This will 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?
Situations such as making a time-lapse of the night sky are a bit different. The limited amount of available light will mandate a long exposure time of twenty seconds or so. This will, in turn, limit the rate at which you can take each photo.
When taking photos during the day with plenty of light, the decision is not so simple. The chosen exposure time 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.
How Does It Look with a Fast Shutter Speed?
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 selected a shutter speed of 1/500 second. This freezes the motion of every vehicle in the scene.
There is some sense that construction vehicles are moving smoothly. But the normal traffic movement looks random and manic.
If the shooting intervals were much longer, no 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%.
How Does It Look with a Slow Shutter Speed?
In this example, I’ve taken another 150 photos at the same interval. But I set the shutter speed to one second which is 25% 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
In general, an exposure of around 50% of the shooting interval produces a natural-looking motion blur. For the last example, that would have required an exposure time of two seconds.
Some photographers call the 180 Degree Rule the “Fifty Percent Rule”. It dates back to the days of rotating mechanical shutters on film cine cameras.
These cameras used a rotating shutter that exposed the film for a proportion of a complete revolution. It 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 were given over to exposing the film. Here’s an illustration showing the timing.
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 for the same amount of motion blur as in cinematography.
Time-lapse video produced from a sequence of stills is different from that of a video/cine camera. The sample rate for cine/video tends to be 24, 25 or 30 frames per second.
Applying the 180 Degree Rule would give typical exposure times of around 1/50 second. If you make a time-lapse sequence from stills shot several seconds apart, you often need much slower shutter speed.
In the first example clip, I’d set my camera to a shutter speed of 1/500 sec at f/8 ISO 200. To get down to a shutter speed of one second, I could have reduced the ISO to 100 and then close the lens right down to f/22.
But that would only give me a shutter speed of 1/30 second. Still far too fast (only three degrees of spinning shutters) to record an obvious blur.
Neutral Density Filters
The solution is to cut down the amount of light entering the camera. Like donning a pair of sunglasses. It’s important that the light is not polarised, tinted or filtered.
It must 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 do this by attaching the correct ND filter to your lens.
You might imagine that ND filters are marked 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 e.g. 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 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.
It is very easy to use a 3-stop ND because it’s still possible to see through it to compose your shot. But a 10-stop is almost 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. In manual mode, I set this shutter speed. I then dialled in the correct aperture and ISO for a good exposure.
After checking my focus and framing, I set the shutter speed to one second. I 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 to attenuate light. The angle between the filters determines the amount.
Looking at the markings on this 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 fast. This is because the two polarisers approach an angle of ninety degrees to one another.
This is Malus’s Law. In practice, it makes these filters very difficult to set to a known value.
A neutral density filter should only affect light intensity. But this type of variable filter will select certain planes of polarisation. It thereby alters the image in unwanted ways.
They also tend to introduce uneven attenuation and false colour in full cross-polarisation. If you dial in the maximum setting for a really slow shutter speed you could end up with something like this.
Motion blur in time-lapse photography is a useful technique to master. And good quality (non-variable) ND filters make it much easier.
Use the 180-degree rule as a guideline and enjoy all the creative possibilities. You can combine slow shutter speed and normal frame rates for amazing results.