Optical Image Stabilization (OIS) Explained- Does your smartphone need it?

by Mayank
ois

Imagine you are having a late night party with your friends and colleagues, and that you want to capture every moment on the party. But who carries a DSLR to a party, right? So, you end up clicking pictures from the camera on your smartphone. After clicking 1 or 2 pictures, you find that these photos look blurry. As it turns out, you know something about photography and therefore reduce the shutter speed by using the manual mode of your smartphone camera to quickly capture the image and thus reduce the blur. But here you were presented with another problem-the photos are now dark as the shutter closes really fast, preventing a lot of light from reaching the image sensor. Even tweaking your exposure didn’t help.

The problem is that when you click a picture, your hands shake, making it look blurred. You’d think of bringing a gimble to counter this wobble, but carrying a gimble is kind of a hassle. This is where the OIS (Optical Image Stabilization) technology kicks in.

Optical Image Stabilization (OIS)

As the name suggests, “OIS optically stabilizes an image” This technology has been around in cameras like DSLR & Mirrorless cameras for decades. Today this feature exists in nearly all flagship smartphones. In fact, since 2010 when the smartphone revolution started, the existence of features such as OIS, NFC, Fingerprint security, etc led to the success of flagship smartphones. These features allowed smartphone manufacturers to create a line between budget and flagship smartphones.

Other image stabilization techniques exist such as EIS (Electronic Image Stabilization) & DIS (Digital Image Stabilization) which depend largely on memory and computational power of your smartphone. By contrast, OIS uses dedicated hardware built into the camera module itself to counteract hand movements while taking a picture. As OIS acts directly on the position of the lens itself, it requires less processing and computational resources.

Optical Stabilization is the most effective method of removing blurriness from an image caused by a hand jitter, whether you are capturing a macro shot of a flower, a light trail at night, capturing a view of the night sky or recording a video. This technique is so effective that even telescopes use it to capture the images of deep space.

Now let’s dive a little deeper to understand how OIS works on your smartphone.

Working of OIS

OIS controls the path of the image between the target and the image sensor by moving the mechanical parts in the camera module. To understand it better, please refer to the figure below.

Image Credits: STMicroelectronics

A pixel on an image sensor captures a single cone of light. Many of such light capturing pixels form the final image you see on your phone or digital camera. Now, let’s suppose we take a picture of a non-moving object, and the camera shutter remains open for a specific time, say 1 sec. While the shutter is open, your hand will jitter and, if no stabilization is provided, the single cone of light captured by a pixel will get scattered on that pixel as represented by A-B in Fig. (a).This phenomenon will occur in each and every pixel of the image sensor. The result of this is a blurred image of the object. If OIS is provided, the camera lens moves in the opposite direction to the camera shake and therefore the image matches the actual position of the object as shown in Fig. (b). The result is a blur-free image of the object.

In simple words, the lens movement of the camera compensates for your hand jitter. If you want to dive more deeper into the internals of it then go through the section below. Else, you can skip this section and move to the next.

Note: You need some electrical knowledge to fully understand the next section.

Diving Deeper

The above section outlined the basics of what OIS does. But the problem doesn’t end here. As you may have noticed, your hands don’t just shake in one direction- they shake at the same time in multiple directions. To resolve this problem, we realize the motion of our hands on the three co-ordinate axes (Longitudinal, Vertical & Lateral axes). Consequently, the hardware used to achieve this needs to be more complex.

To make this simpler, let’s divide an OIS-enabled camera module into its various mechanical and electronic components:

Mechanical Components

The free movement of the lens in all directions is necessary in order to implement OIS on your smartphone. In order to achieve this, an actuator is required. The most widely used actuation is the Voice Coil Actuation. Th consists of a current-carrying coil winding and a permanent magnet connected to the sides of the camera module housing to face each other. The magnetic field is produced by the current-carrying coil when current is applied to it. This field interacts with the magnetic field generated by the permanent magnet. This interaction produces a force which moves the camera lens. The distance moved by the lens is directly proportional to the current applied to the coil.

The architecture used to build the camera module is called barrel-shift (or lens-shift) in which the image sensor is fixed at the bottom of the module housing and the lens moves on top of the sensor through the Voice Coil Actuation explained above.

Lens-shift or Barrel-shift architecture. Image Credits: STMicroelectronics

So, what we now have is a camera module that houses a lens which can travel in either direction. We’ve hit the halfway point of knowing OIS on smartphones. What we need now is a signal to control the movement of the lens precisely. Here we get to the electronic components required to completely implement OIS.

Electronic Components

The electronic circuitry used to implement OIS consists of 2 main components. These components are:

  • Gyroscope: The gyroscope is used to sense vibrations or movement of the device while clicking the picture. The gyroscope measures angular movements of the camera which are caused when your hands shake while clicking a picture. These movements are referred to as- Pitch, Yaw & Roll. (The x, y and z co-ordinates) as shown in figure below.
An OIS-enabled camera module showing different axes of rotation of the lens

Note: The gyroscope works only on 2 dimensions at a time i.e. if the OIS is working to compensate the pitch and yaw movements, then it cannot correct the roll movement simultaneously.

The gyroscope provides a reference signal to determine the angular movement of the device. It is placed in a flexible PCB which is connected to the main camera module.

  • Hall sensors: These sensors are directly integrated in the housing of the camera module to get fast and precise angular measurement of the correct position of the camera lens.

Through a closed control loop, an error signal is first generated by comparing the reference signal from Gyroscope and the signal from Hall Sensor. This error signal is sent to a microcontroller (which is also integrated in the flexible PCB along with the gyroscope) and this microcontroller sets the new position for the VCM actuators. A driver then moves the lens to compensate for the hand jitter. This is how the Optical Image Stabilization is implemented in smartphones.

The complete OIS-enabled camera module. Image Credit: STMicroelectronics

Benefits & Limitations of implementing OIS on smartphones

Benefits:

If you have reached this stage then you can now see the advantage of using OIS. This technique enables you to take really close shots of something like a flower & decent shots at night. Since it also works on video, you’d have better stabilized frames while capturing videos, especially those where you record while walking like Vlogs. If your smartphone has OIS, you can tweak shutter speeds and exposure times to get your best picture without worrying about your hands wobbling.

Limitations:

  • The addition of OIS makes the camera module bulky, which in turn adds to the weight of the telephone itself.
  • The camera modules on smartphones are very small and usually only cover 100mm2. The fitting of mechanical parts as well as electronic circuitry in such a small area is a major engineering challenge that makes it expensive.This is the main reason you find OIS mostly on flagship smartphones.
  • Another drawback to this technology is that while it can stabilize the image blur caused by the photographer’s hand trembling, it cannot stabilize the blur caused by the objects moving too fast out of the camera frame. For example in the below picture you can see that the car moved too fast for the camera’s exposure to capture it. In such cases, image stabilization won’t help you.
In this image, the car moved too fast for the camera to capture it. In such cases OIS won’t help to reduce the blur from image

Conclusion:

If you are buying a smartphone for photography, optical image stabilisation is a must have on your device. While this technology is limited to flagship phones, advancement in electronics will one day make OIS available for budget phones too and that time is not far away.

So, what type of image stabilization does your smartphone have? Let me know in the comments below.

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