Many modern lenses have a feature called image stabilisation (IS) that is often touted to give the photographer the ability to take sharp images at slow shutter speeds that on any ordinary lens, would cause a blurry result. It sounds great but how effective is this technology? How does it work? What are its limitations and how can we get the best out this technology?
Today’s blog entry on Image Matters explores the wobbly world of camera shake.
The basic blur problem
Blur is caused by the image moving across the sensor during the exposure. This is usually a combination of the subject movement and the camera movement. Provided your exposure time is short enough, the blur can be kept to less than a pixel and so the resulting image will appear sharp (assuming it’s in focus). Very short exposure times need plenty of light for good exposure and so lenses that allow you to use fast shutter speeds need to be able to admit a generous amount of light – which equates to lots of expensive glass and a wide maximum aperture of f/2.8 or wider. In a situation where there isn’t plenty of light or you just can’t open the lens wide enough for whatever reason, a longer exposure is the only answer (unless you’re happy to keep cranking up your ISO and don’t mind the associated increase in image noise). Even if your subject is perfectly stationary, hand holding the camera on a long exposure is almost certain to produce some blur due to your inability to hold the camera perfectly still.
Shake rattle and roll
We all shake to some extent all the time. The neuromuscular tremor of a healthy person is a low level, roughly sinusoidal vibration at a frequency of around 10Hz. Most of the time, we don’t notice it unless we’re suffering from stress, fever or trying to carry out a particularly delicate operation. As photographers, we notice it most when trying to hold the camera steady whilst holding a heavy lens with a narrow field of view. There are three main ways we can inadvertently wave the camera around – pitch, yaw and roll.
The 1/f rule
Until the digital era, the only solution to camera shake when not using a tripod was to ensure a sufficiently short exposure but just how fast does it need to be?. A general rule-of-thumb that’s well known to many photographers is the ‘1 over f rule’. This says the slowest shutter speed you should use is about one over the focal length of the lens. For example, using a 50mm lens hand-held at anything faster than 1/50th second should be fine but this would be too slow for a 200mm lens where a minimum shutter speed of 1/200 second would be indicated. The problem is that sometimes we might, for example, be using our ‘nifty fifty’ lens at full aperture and the camera’s metering system says we need to be shooting at say 1/8th second which is far too long for hand holding. What can be done if we don’t want to up the ISO any further?
Given a situation where the image is wobbling around on the sensor during the exposure as a result of the photographer being unable to hold the camera still enough, it turns out that there is something that can be done to calm things down. That something is what engineers call an open-loop control system. The idea is simple enough in concept but as it turns out, is somewhat limited in its ability to avoid every blurry image. A sensor in the camera detects the amount and direction of movement and attempts to compensate by an equal and opposite amount to cancel out the effects of the movement – rather like noise cancelling headphones generate sound intended to cancel out the unwanted sound from outside world. This compensating signal can be made to either move the sensor itself or the image falling on it. Nudging the sensor seems like a good idea but of course there’s a limit to how far you can move it. One advantage of this system is that since the mechanism is part of the camera, it works with any lens so long as the camera knows the focal length.
Moving the image itself and keeping the sensor where it is provides a greater range of correction but the mechanism for moving the image has to reside in the lens and this is the method that is used in Canon lenses. A sensor in the lens detects the camera movement and a computer (also in the lens) uses this information in conjunction with the focal length, to calculate how that movement would have affected the image on the sensor (it can’t actually read the sensor as it’s hidden behind the mirror – making it an open loop rather than a close-loop control system). It then adjusts some compensation optics that shift the image by that amount in the opposite direction. The net effect (in theory) is that the image doesn’t move as the camera shakes. This is no mean feat and it requires quite a lot of electromechanical gubbins to be crammed into the lens body as shown below. This extra complexity of course adds to the weight, size and cost of the lens.
The Canon IS system attempts to correct for both vertical and horizontal shake but cannot correct for rotational movement. This is not really a problem in practice since most movement is vertical (pitch). The main limitation is connected with what’s called the frequency response of the system. In other words, it doesn’t work equally well at all vibration frequencies. Remember that the camera has to be able to detect a movement, work out what correction is needed and then control some actuators that move the compensation optics. This can’t be done instantaneously so the timing itself is critical. If the system reacts too fast, the optics will jitter and could even become unstable. Too slow, and the image will begin to move across the sensor before the IS system can detect and correct it. Some of Canon’s newer lenses are capable of quite impressive performance when it comes to IS but you need to be aware of their limitations. Typically, the IS efficacy is stated in f-stops. For example, without IS switched on, you might be able to get a sharp image hand held at 1/250 sec but no slower. The same lens with IS switched on and having a range of 3 stops would enable you to get a similarly sharp image at a shutter speed corresponding to 3 stops less or 1/30 sec. Some lenses even claim 4 stops of IS but beware – such impressive performance is generally only found at a vibration frequency range of up to 10 cycles per second (Hz). At higher frequencies, the effectiveness of the IS begins to drop until a point is reached where it’s not really doing anything but consuming power.
A quick and dirty test
To test this, on a typical lens, I set up a rather makeshift vibration rig cobbled together from a guitar amplifier and a mobile phone running a signal generator app. The camera was suspended by the strap to allow it to pitch in response to sinusoidal vibrations from the amplifier’s dismembered speaker transmitted to the front of the lens via a plastic tube. Since the amplifier wasn’t DC coupled, the lowest frequency I could use was 20Hz. This is faster than your hand is likely to vibrate but slower than the vibration you might get, for example, from a car engine. The camera was focused on a test target and remotely triggered at different shutter speeds with the IS switched on and off. The amplitude was adjusted to generate the same amount of blur one might expect by hand holding. The focal length in this instance was 200mm and a shutter speed of 1/200 sec gave sharp results as expected using the 1/f rule with or without IS switched on. One stop slower at 1/100 sec showed camera shake blur without IS but sharp results with IS but at two stops (1/50 sec), the image was equally blurred with and without IS. At lower frequencies, we might have had a sharp image down to 1/25 sec (as this lens claims three stops of IS) but at 20Hz, it could only manage one stop. Although these tests were not by any means exhaustive, they did demonstrate that the effectiveness of image stabilisation decreases as the frequency increases. This is never mentioned in the advertising material but it’s exactly what you’d expect. As the frequency increases, there comes a point where there simply isn’t enough time for the compensation system to react.
Getting the best results from IS
If you set the IS switch on the lens to ‘on’ it doesn’t immediately activate the IS system. If it did, it would be continually draining extra power from your battery. Instead, the IS system is activated only when you half press the shutter release button and it stays working until you take the photograph. As we’ve noted, the IS system has a finite reaction time and limited useful bandwidth but it also has a startup delay. If you look through the viewfinder when hand holding a long focal length lens for example, you’ll see how shaky your hand really is. Then, if you gently press the shutter release half way, you will see there is a noticeable delay before the image settles down and the shake is reduced.
The upshot of this delay is that quickly stabbing the shutter release, as some photographers are prone to do, not only causes extra camera shake but doesn’t actually give the IS system time to stabilise the image before the shutter is released. If you want to get the sharpest images possible, gently activate the shutter release button half way and then take the shot. If you are taking a rapid burst of shots, the IS system will still need time to activate for the first exposure but once the sequence begins, it will remain active for the other shots in the sequence. This is why the first shot of a burst is sometimes not as sharp as the following exposures (another factor is how the auto focus system is set up but that’s another article).
Some lenses just have a simple IS on/off switch but others have an IS mode switch. The reason for this is that by default, the Canon IS system helps to reduce the effects of both pitch and yaw but there are times when this can be more of a hinderance than a help. A notable example is when following the motion of an subject by panning the camera. This can be a tricky skill to master but it can be made all the more difficult if your IS system is trying to work against you to minimise the effects of your deliberate horizontal camera movement. In a situation such as this, it’s useful to allow the camera to compensate for the effects of vertical camera shake but not horizontal shake. This is what MODE 2 is for. If you want IS in both horizontal and vertical, select MODE 1.
Myths about IS
IS will help to make all your images sharper.
No – it won’t make any difference to images where the subject is moving too fast or the focus is not correct. Nor will it make any difference if your camera is on a tripod. It’s only a benefit when hand holding the camera in conditions that demand a slower shutter speed than the 1/f rule would recommend.
With IS, you don’t need such a large maximum aperture
Having a small maximum aperture reduces the maximum amount of light your lens can admit to the sensor compared to a lens with a large maximum aperture. This in turn means that you are sometimes forced to use slower shutter speeds. If you’re also hand holding the camera, this is where IS can be a big help. However, IS doesn’t help with respect to subject movement and there are depth of field considerations too. It’s always good to have a wider maximum aperture at your disposal as this allows a faster shutter speed to reduce blur due to subject motion and also allows you to select a narrow depth of field. If you also have IS, then that’s ideal!
Switch IS off when not using it to save your battery
Don’t worry about switching IS off to save your battery. Firstly, IS is only activated when you half press the shutter button and secondly is only consumes about an extra 30mA of current which is minimal in comparison to the extra current taken, for example, when using Live View.
Using a slower burst rate will give sharper results
You may see some people suggesting that since the IS system takes time to kick-in, that you should not shoot at the fastest rate when using burst mode. The reasoning is that using a slower burst rate in continuous shooting mode gives the IS system time to stabilise the image between exposures. This is nonsense. The IS system has a small time lag between the shutter button being half pressed and being fully operational but once a burst is in progress, it remain active until the end of the sequence. There would be no point in switching it off between exposures in burst mode.
Using IS when the camera is on a tripod can damage your lens
If you activate IS when the camera is fixed on a tripod, it can introduce some unintentional hunting of the correction optics and actually create the effect of camera shake instead of correcting for it. For this reason, Canon advise that IS is turned off when using the camera on a tripod. However, there is no way that this effect can damage your lens or camera. Some of the more recent lenses can detect when the lens is absolutely stationary and automatically switch the IS system off.