Image stabilisation performance is usually described in terms of shutter speed stops, such as ”equivalent to 5 shutter speed stops” or ”4 stop equivalent”. How are these numbers measured? Read on to find out more. (Reported by: Ryosuke Takahashi)

 

The effects of IS

What is camera shake?

“Camera shake” refers to blurring in the image caused by the movement of the camera, including movement caused by unsteady hands.

Why is image stabilisation important?

As a rule of thumb, if you want to shoot handheld, your shutter speed has to be a at least 1/focal length seconds to prevent camera shake.

However, it is not always viable to shoot at 1/focal length. For example, in dark shooting conditions, you might need to use both your maximum aperture and a slower shutter speed to ensure that the shot is sufficiently exposed without too much image noise.

Image stabilisation (IS) helps to correct the camera shake so that you can shoot at a slower shutter speed with little to no visible camera shake blurring. (Note: It cannot correct the blur caused when the subject moves!)

IS On

IS Off

EOS R/ RF24-105mm f/4L IS USM/ FL: 105mm / Manual (f/4, 1/4 sec) / ISO 800/ WB: Daylight

In this example, the focal length was 105mm but the shutter speed was 1/4 second, which is about 4 shutter speed stops slower than the 1/focal length rule. This results in visible camera shake when IS is off.

What does “IS effect up to XX shutter speed stops equivalent” mean?

It means that when you shoot with IS on, the camera shake correction effect looks like you were shooting at a shutter speed that is up to XX stops faster.

In the above example, the RF24-105mm f/4L IS USM has an IS performance equivalent to approximately 5 shutter speed stops. This suggests that the “IS On” example is just as sharp as if it were taken at around 1/100 to 1/125 second.

 

How IS stops are measured

The effective number of IS stops is measured and determined using methods established by the Camera & Imaging Products Association (CIPA).

This link has more information on the methods, but here’s a summary:

Procedure:

1. The equipment to be tested is set onto a vibratory apparatus.

2. It is then subjected to either one or two types of mechanically-created vibration waveforms, depending on the total mass of the camera body, lens, memory media and battery.

3. During vibration, the camera is used to shoot a motion blur measurement chart with the image stabilisation turned on.

Generally, a total of 10 shots are taken at shooting intervals of approximately 1 second. Shooting is stopped after 30 seconds even if less than 10 shots are taken.

4. At least 200 shots of the motion blur measurement chart are taken under the same conditions (shutter speed, focal length, shooting distance etc).

5. The shutter speed is increased or reduced by at most one stop, and Steps 3 and 4 are repeated.

6. The entire process from 1. to 5. is repeated until two measurement results are obtained:
i) The slowest speed where the blurring does not exceed a predetermined benchmark (also known as the “determination level for image stabilisation performance”), and
ii) The fastest shutter speed where the blurring is more than the predetermined benchmark.

7. The results of the shots are determined using a dedicated blur measurement software, and the effective number of stops indexed according to the calculation method described below.

Know this:

1) To prevent any fluctuations arising from differences in the technical skill of the photographer, IS performance is not measured using actual handheld shots.

2) In practice, camera shake is difficult to suppress when shooting with shutter speeds as slow as 30 seconds. The slowest shutter speed where the IS system is effective is unknown and probably differs between cameras. Canon has said that their DSLRs “are able to operate at low shutter speeds of up to 30 seconds”.

3) The conditions created by the vibratory apparatus differ from actual camera shake, so the published number of IS stops might not completely match what a user experiences when using the camera.

 

Method for calculating IS performance based on the CIPA standard

A: IS performance (shutter speed stops)
B: Reference motion blur amount
C: Measured motion blur amount
D: Determination level for image stabilisation performance

Source: Figure 4-5-3b Image Stabilisation Performance Calculation Method, from “Measurement and Description Method for Image Stabilisation Performance of Digital Cameras (Optical System)” published by CIPA.

What the terms mean

- Reference motion blur amount (B): A value that represents the amount of blurring without IS.

- Measured motion blur amount (C): A value that represents the residual motion blur when IS is turned on. This is derived from the results obtained with the vibratory apparatus.

- Determination level for image stabilisation performance (D): It is difficult to determine whether any image stabilisation took place if very little blurring occurred. This value represents the predetermined level of blurring that is just enough for the CIPA panellists to know that image stabilisation was performed.

Calculating the amount

1. Identify the points on B and C that intersect with D.

2. Identify the shutter speeds (horizontal axis) where these intersections occurred.

3. The effective IS performance in terms of shutter speed stops is calculated from the difference between the two intersections.

 

Examples of Canon lenses with built-in IS

Approximately 5 stops

EF70-200mm f/4L IS II USM

Approximately 4 stops

Note: IS speeds listed are possible only with compatible cameras.

 

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