CMOS SLR inside

Using Image Stabilisation

Image stabilisation, in still photography, refers to a lens or camera’s ability to limit the effect that camera movement has while taking a picture. Unless on a tripod, if an exposure takes a long time (anything from 1/100th of a second to longer than a second), then your hands will move the camera while it is taking the picture. Hey presto – unwanted camera shake!

Canon usually refers to the technology as Image Stabilisation, whereas Nikon usually refers to it as Vibration Reduction (but I prefer the acronym VR to be used for Virtual Reality). Canon and Nikon (as well as other camera manufacturers) have lenses that stabilise an image by moving a group of lens elements (this is optical image stabilisation – it can also be done by moving the image sensor, which some smaller cameras and phones do). Digital Image Stabilisation is mainly used to stabilise video, and is not really sited to still image capture. I shall refer to all shake-reduction technology as IS.

How much do you need IS?

The necessity of IS depends on four things:

  • How much light is available/Aperture/ISO
  • Focal length
  • Sensor size/Resolution

The amount of light available – whether natural or flash – is a big factor. The less light you have (or if you have a small aperture / low ISO), the longer your shutter speed will need to be.

Longer focal lengths will have more pronounced camera shake.

Sensors with higher resolutions will record even tiny camera shake (e.g. the 50mp Canon Eos 5Ds). APS-C sensors at the same megapixel count as full-frame sensors will record twice as much camera shake (and therefore require shutter speeds that twice as fast), simply because their pixels are more densely packed together.

 

The Reciprocal Rule

This is more of a rule-of-thumb, but it’s useful as a starting point. The ‘rule’ states that, on a full frame SLR camera, your shutter speed as a fraction of a second must be greater than focal length. For example, to eliminate camera shake at a 50mm focal length, your shutter speed must be faster than 1/50th of a second.

Cameras with APS-C sensors require speeds at least double full frame (so a speed of 1/100th of a second would be necessary to freeze shake on a 50mm lens). This is not too much of a worry, as 25/30mm on APS-C is equivalent to 50mm lens on a full frame camera, so while you may need faster shutter speeds for the same focal lengths, the focal lengths you will be using will be shorter. This ‘rule’ is not infallible, but is a good guide to the shutter speed required to eliminate camera shake when taking shots handheld.

 

2, 3, and 4 stop Image Stabilising

Sometimes, you’ll need longer exposures – either to intentionally capture movement, or because you simply don’t have enough light at your chosen ISO and aperture. This is where IS comes in. Each stop of IS your lens/camera has allows the shutter speed to be halved. So 2 stops of Image Stabilisation allows your shutter speed under the reciprocal rule to be divided by 2, twice. For 3 stops, divide by 2 three times. Using our previous example, a shutter speed of 1/8th of a second would be required to freeze camera shake with a 50mm lens (for those not good at maths, that means divide by 8). 4 stops of IS would allow a speed of one quarter of a second.

 

‘Up To’ 4 Stops… (Also Beyond)

Manufacturer’s specifications usually include the words ‘up to’ when listing the IS capabilities of a lens. This is because it is impossible to freeze all hand movements all of the time. At a lens’ maximum IS specification, some of your shots will be sharp, some won’t.

Since the percentage of sharp shots is what you are determining, you can choose how far you dare push your shutter speed. Sharp shots that are slower than the max IS capability of your equipment ARE possible if you shoot enough frames of the same picture. Let’s say you have a 200mm lens with 3 stop IS – the slowest shutter speed you should be able to shoot at and still retain a reasonable percentage of sharp photos is 1/25th. However, if you take 10 shots at a 1/8th, at least one will be sharp. This is due to the motion of your hands – as camera shake is the result of swaying left to right (or up and down), then there is a time when hand movement stops at the far right and left (or top and bottom) of the sway. Think of a pendulum swinging back and forth, there is a brief moment when it stops between each swing as it reverses direction.

If you find yourself needing to push the limits of your IS system, my advice would be to first shoot a few shots with a shutter speed that you know is fast enough to freeze camera shake, as safety. You then know that you’ve got at least a couple of usable shots. Next, drop your shutter speed for subsequent shots to allow a lower ISO, or greater depth of field if you need it. You’ll sometimes be surprised just how far you can push. The lower ISO images will be less noisy, but fewer of them will be sharp, so go snap happy because you’ll have a lot of blurry throw-away images. In the end, if you get a great shot out of it, it’s worth it.

 

P.S. As a footnote, Tony & Chelsea Northrup have a great YouTube video discussing shutter speed which is worth a watch. Skip to 19min 30sec for Image Stabilising tips.