Image Quality Factors

Image Stabilization

Image Quality Factors
  1. Introduction
  2. The background of image stabilization
    1. Focal lengths
    2. Low light
    3. Preventing blurred images with shorter shutter speeds
    4. Setting a short shutter speed
  3. Image stabilization methods
    1. Tripods
    2. Optical image stabilizer
    3. Sensor-shift optical image stabilization
    4. Other image stabilization methods
  4. Measuring the quality of your image stabilization system
  5. Conclusion
  6. References

Introduction

Image stabilization refers to how stable the optical system within a camera is during the image capture. If the camera is not stable, then the image will appear blurry. There are many causes of blurred photos such as weak lighting conditions, the use of long focal lengths, and slow shutter speed in combination with camera shake from a hand-held camera.

The background of image stabilization

Focal lengths

The focal length is the distance between the main level of a lens system and the image focal point for infinity. Depending on the size of the recording area it determines the angle of view.

A wide-angle camera, for example, has a short focal length meaning the focal point moves closer to the main level. With a short focal length, the angle grows, the receiving area widens, and the subject in the image is displayed smaller. For correct exposure, the camera sensor must capture a certain amount of light.

Low light

If in a low light situation, the light must be adjusted to achieve the correct exposure. This adjustment is made by adjusting the aperture, ISO sensitivity, and shutter speed. So, if there is a low light situation, a higher amount of light can be achieved by choosing a larger aperture, a higher sensitivity, or a slower shutter speed. The darker it becomes; however, the fewer possibilities are available.

There are many situations where the max aperture is not enough and increasing sensitivity, will have too great of a negative impact on image quality. In this circumstance, lowering the shutter speed is the only choice.

Using a longer exposure can help with correcting exposure in low light situations, however, the slower the shutter speed, especially with a hand-held camera, the more the camera vibrations will potentially be captured and lead to blurred images.

Preventing blurred images with shorter shutter speeds

As a general rule, the shutter speed should be at least as short as the reciprocal of the focal length.

T = 1/focal length

To avoid image blur from a hand-held camera, it is important to keep in mind that the longer the focal length (35 mm equivalent focal length), the shorter the selected shutter speed should be. Using a focal length that is too long will lead to an increase in visible vibrations.

shutter speeds
Image 1: A comparison of ISO 100 - f/5.6 - 1/13s - 200mm (left) with ISO 400 - f/5.6 - 1/50s -200mm (middle) and ISO 1600 - f/5.6 - 1/200s - 200mm (right).

Setting a short shutter speed

For most situations, short shutter speed can be set by selecting a larger aperture or a higher ISO sensitivity in the camera settings. Oftentimes, however, the camera does not provide the proper settings concerning a photographer’s preferences. For example, the photographer may need a smaller aperture number than what is available. To combat this problem, a higher ISO sensitivity can be selected, but doing so often risks amplifying the image signal to a level that increases the noise and lowers the image quality. If a shorter shutter speed cannot be achieved using the camera settings, other stabilization methods outside of the settings can be utilized for blur-free images.

Image stabilization methods

It’s important to understand that short shutter speed is the best solution for capturing a fast-moving object. Without a short shutter speed, the motion of the moving object will result in image blur even when using the stabilization methods listed below.

with motion blur
without motion blur
Image 4: Movement during exposure leads to blurring (left). When there is not movement the image can be captured sharp (right)

Tripods

The classical way to prevent blurring through slow shutter speed is by using a tripod. Of course, a tripod is not perfect and much of its effectiveness is based on how sturdy the surface it’s situated on is. Additionally, a manual release can cause slight camera shaking, so a self-timer or a camera remote control is recommended.

As reliable as tripods are, they cannot be used in all situations. As a result, most cameras and lenses today have built-in image stabilizers either based in the lens (In-Lens) or sensor (In-Camera) that can compensate for stabilization issues such as hand trembling.

Optical image stabilizer

An optical image stabilizer in the In-Lens method contains a floating lens element with corresponding sensors that counteract vibrations. Essentially, small measuring instruments register the movement caused by the photographer and forward the information to a processor that then creates a corrective adjustment with the help of the floating element. The element can be moved in both horizontal and vertical directions to balance the yaw (rotation on the Y-axis) and the pitch (rotation on the X-axis).

Optical stabilization
Image 5: The rotation around the Y-axis is now as yawning or yaw and the rotation around the x-axis is known as pitching or pitch.
Optical stabilization
Image 6: The floating lens element (green) moves in a horizontal and vertical direction to compensate for the rotation movements around the x and y axes (right).

The optical In-Lens image stabilizer is particularly significant in cameras that have an optical viewfinder (e.g., telephoto lenses). If the stabilizer is active, it begins working as soon as the shutter button is pressed halfway. From there, the stabilized light information is passed through the camera’s internal mirror to the optical viewfinder allowing the subject to be calmly set in the scene.

The optical In-Lens image stabilizer method is one of the more common image stabilization methods, but it is not available for every lens.

Sensor-shift optical image stabilization

Some manufacturers rely on a sensor-based (In-Camera) stabilizer. In comparison to the In-Lens stabilizer where the sensor is fixed, this method uses a free moving sensor to atone for camera shake. In other words, the sensor is shifted in the required direction along the x or y-axis depending on the camera movement. In addition to stabilizing the movements along the x and y axes, sensor-shift stabilizers can also offset the rotation along the optical axis (roll), thus eliminating camera shake from three separate axes.

Sensor stabilization
Image 7: A sensor-shift stabilizer is able to correct camera shaking along the yaw, pitch, and roll axes.

It is important to note that both the In-Camera and the In-Lens stabilizer can be integrated into the camera and lens system at the same time. In this instance, many cameras select one or the other but some cameras use both at the same time. In these cases, the manufacturers have to make sure that they do not negatively interfere with each other and ultimately lead to unwanted blurry images.

Other image stabilization methods

Whereas the before mentioned image stabilization methods are called (purely) optical image stabilization there are additional methods that often include “non-optical” aspects. For example, some manufacturers of cellphones use methods where they combine multiple underexposed images with shorter exposure times to a regularly exposed image by registering the images to each other and combining the signal levels to a final image. Another method is to combine a regularly exposed blurry image with the edges taken from an underexposed sharp image. One of the difficulties with these methods is defining “the” exposure time for the image that gets reported. Even with all of that said, the only thing that matters to the user is getting a sharp image even in low light conditions.

Measuring the quality of your image stabilization system

One of the most effective methods for measuring the image stabilization quality in your camera system is using a shaking simulation device and test charts. This method is described in ISO 20954-1.1 With this type of device, you can define and control the camera shaking to produce comparable results. Once the device is set, the camera can capture images of a test chart with sharp images and other potential structures at various exposure times. An evaluation of the edges will report which exposure times will produce blurry edges.

STEVE setup
Image 8: An example setup using the STEVE stabilization simulation device and a slanted edge test chart.

If the exposure time cannot be controlled, the process gets slightly more complicated. For example, when the illumination level on the test chart is modified (reduced), the camera is forced to react by either opening the aperture, raising the ISO speed level, or increasing the exposure time. Raising the ISO speed will lead to higher noise levels or a loss (noise reduction) of low contrast fine details also known as texture loss. An increase of the exposure time will again lead to blurry edges in the absence of an image stabilization method. For these situations, the test chart will need to be more complex with targets for analyzing noise and texture loss.

TE42-LL
Image 9: An example test chart with detailed targets for analyzing noise and texture loss.

Conclusion

Image stabilization refers to how stable an optical system of a camera is during capture. Without a stable system, images will appear blurry and affect the overall image quality. Methods such as using a tripod, an optical stabilizer, or sensor-shift optical image stabilization can all help improve image stabilization, but it is also important to test the quality of the camera's image stabilization capabilities.

When testing the camera, we recommend following ISO 20954-1 and using a slanted edge test chart and a camera shaking simulation device. This device will allow you to define the level of shake and then you can easily test various exposure times with the slanted edge test chart and see which ones lead to the most blurring. For low light testing, we advise using a chart that also has targets for measuring noise and texture loss.