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IS – Secret to Reducing Camera Shake

Among Canon's EF lenses, those marked with the "IS" wording are built in with the Image Stabilizer, a feature that helps to reduce camera shake. It has been more than 15 years since the release of the first EF lens with an IS feature. However, some aspects of this feature, such as the availability of different IS modes and control of its operation, remain relatively unknown to many users. In this article, I will shed light on the functions of the IS besides its ability to reduce camera shake.

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Differences between in-lens and in-body IS

Generally, camera shake is said to occur during handheld shooting at a shutter speed that is slower than "1/focal length" seconds. Shutter speed may slow down such as when you are photographing at a dark location or under low light, and camera shake would be inevitable if you are taking a handheld shot. The technology that Canon developed to address this issue is the IS feature. When the lens is tilted due to camera shake, light (image) from the subject goes out of alignment with respect to the optical axis. The basic mechanism of the IS is to stabilise the image by moving a part of the optical system within the lens (corrective optics) according to the amount of shake. As correction is performed inside the lens, this is referred to as the in-lens IS system. In contrast, shake compensation that is performed by moving the image sensor in the camera body is known as the in-body IS system. Many users are unaware of the differences between these two systems.

Image that passes through the lens is directed inside the camera as illustrated in the diagram. In the case of the in-lens IS system, the corrected image is reflected by the main mirror and directed to the viewfinder and AF sensor before exposure, after which the main mirror flips up and the shutter curtains open during exposure for the image to reach the image sensor. Meanwhile, the in-body IS system performs correction by moving the image sensor according to the amount of shake. Both systems correct shake on the recorded image, but differ in the state of the image that reaches the viewfinder and AF sensor up to the point when the shutter is released. As shake is corrected inside the lens in the case of the in-lens system, the image is stabilised before it reaches the viewfinder, thereby delivering a clear viewfinder image and enabling easy and accurate framing. Similarly, the image reaching the AF sensor is also stable, and this helps to enhance the stability and speed of AF computation particularly in a low-light situation. Additionally, the in-lens system also increases the flexibility in adopting the optimal optical design according to each lens. The various advantages that the in-lens IS system offers during the actual shoot is the reason why it is widely supported by many users.

In-lens IS System

Before releasing shutter

A: Viewfinder

B: Mirror

C: Corrective Optics

D: AF Sensor

E: Image Sensor

When shutter is released

A: Viewfinder

B: Mirror

C: Corrective Optics

D: AF Sensor

E: Image Sensor

In-body IS System

Before releasing shutter

A: Viewfinder

B: Mirror

D: AF Sensor

E: Image Sensor

When shutter is released

A: Viewfinder

B: Mirror

D: AF Sensor

E: Image Sensor

In an SLR camera, light (image) that passes through the lens is reflected from the semi-transparent main mirror, and reaches the viewfinder through the prism. The image that passes through the main mirror is reflected from the sub mirror behind before reaching the AF sensor. Upon releasing the shutter, the mirror flips up for the image to reach the image sensor. The key difference between the in-lens and the in-body IS system is that the image is corrected before it reaches the mirror.

IS Modes and Control of IS

Currently, the IS system adopted by EF lenses can be largely divided into two types. The first is the common IS for correcting angular shake (tilts). The other is the Hybrid IS that is also capable of addressing shift shake, which has a more pronounced impact when you are taking macro shots. The Hybrid IS feature is only available on the EF100mm f/2.8L Macro IS USM at present. The IS feature on EF lenses comes with three modes, each operating in a different way to correct camera shake. The most effective choice during general photography is Mode 1. Meanwhile, Mode 2 is suited for panning shots, with shake correction stopped only in the direction of the panning shot when panning by the lens is automatically detected. Mode 3, which can be found on the latest telephoto lens, is intended for users who are aiming at a subject with unpredictable movement, such as in a sports event. When the composition is altered or when panning is performed suddenly in Mode 1 or Mode 2, the initial operation of the IS feature may cause momentary shock to the viewfinder image. To prevent this from occurring, Mode 3 has been developed, which performs shake correction only during exposure after the shutter is released, instead of correcting the viewfinder image. This mode is suited for professional users who wish to have a firm hold of the camera when a telephoto lens is attached without having to worry about such momentary shock in the viewfinder image.

Some lenses come with a switch for selecting an IS mode, while there are some others that switch between the IS modes automatically. Most of the lenses for advanced amateurs and professional photographers come with a mode selector switch, while those for general users are equipped with an On/Off switch for the IS feature. However, note that even for lenses that only come with an On/Off switch, some are able to detect panning and switch to operate in IS Mode 2. You are recommended to read up the lens manual to check whether your lens supports such a function.

There are many users who are not sure whether they need to turn off the IS when the camera is attached to a tripod. In fact, since the release of the EF300mm f/2.8L IS USM in July 1999, all EF lenses built in with the IS feature are equipped with a detection function that disables the IS feature automatically when the camera is mounted to a tripod. For lenses that come with a mode selector switch, you may need to switch the mode according to the circumstances. Nonetheless, for lenses using the latest IS feature, it can be left on at all times unless the surrounding conditions require you to disable it.

Configuration of the IS Unit

A: Direction in which the corrective optics moves

B: Circuit Board

C: PSD

D: Magnet

E: IRED

F: Corrective Optics Lens Barrel Holding Pin

G: Yoke (Magnetic Flux Plate)

H: Corrective Optics

I: Corrective Optics Lens Barrel

J: Guide Shaft

K: Corrective Optics Lock Mechanism

The amount of displacement by the corrective optics for IS is calculated by two ultra-small vibration-detecting gyro sensors (PSD) that detect camera shake in the vertical and horizontal directions in the form of angular acceleration. When the shutter button is half-pressed, the lock for the corrective optics is disengaged. Camera shake is detected by the vibration-detecting gyro sensors (PSD), and the detection signals are sent to a microcomputer where they are converted into vibration signals for the corrective optics. The vibration signals are then transmitted to the drive circuit to move the corrective optics.

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