Gamma, Chart Contrast and MTF Calculations

A fuller and more up-to-date explanation of gamma,
including how to obtain it for slanted-edge MTF calculations

can be found on Gamma, Tonal Response Curve, and related concepts.

Gamma (the average slope of log pixel levels as a function of log exposure for light through dark gray tones) is used, per the ISO 12233 standard, to linearize the input data, i.e., to remove the gamma encoding applied by the camera or RAW converter. Gamma defaults to 0.5 = 1/2, which is typical of digital cameras, but may be affected by camera or RAW converter settings. 

Imatest Settings

For accurate calculation of edge-SFR from using Imatest’s SFRplus, eSFR ISO, SFRreg, Checkerboard, or SFR modules, it’s important to use a proper gamma for sharpness calculations.

If the edge contrast is known and is ≤ 10:1 (medium or low contrast), you can enter the edge contrast in the Chart contrast (for gamma calc.) box, then check the Use for MTF checkbox. Gamma will be then be calculated for each individual edge in the chart image and displayed in the Edge/MTF plot. Full details, including the equation, are on Gamma and MTF Measurement.

Measuring Gamma

If chart contrast is not known you should measure gamma by obtaining the OECF (Opto-Electric Conversion Function) per the ISO 14524 standard.  This can be obtained using an image of a grayscale stepchart and running Colorcheck, Stepchart , Multicharts (interactive), or Multitest. A nominal value of gamma should be entered, even if the value of gamma derived from the chart (described above) is used to calculate MTF.

Errors in Gamma

Small errors in gamma have a minor effect on MTF measurements (a 10% error in gamma results in a 2.5% error in MTF50 for a normal contrast target). Gamma should be set to 0.45 when dcraw is used to convert RAW images into sRGB or a gamma=2.2 (Adobe RGB) color space. If gamma is set to less than 0.3 or greater than 0.8, the background will be changed to pink to indicate an unusual (possibly erroneous) selection.

Technical Details

Capture One LE with Film Curve
1. Capture One LE set to Film standard
(the default). Gamma = 0.679.

Capture One LE with Linear Curve
2. Capture One LE, Linear response. Gamma =
0.508. Recommended for SFR runs.

Canon FVU with Standard contrast

3. Canon FVU set to Standard contrast.
Gamma = 0.642.

Gamma is the exponent of the equation that relates image pixel level to luminance. For a monitor or print,

     Output luminance = (pixel level)gamma_display

When the raw output of the image sensor, which is linear, is converted to image file pixels for a standard color space, the approximate inverse of the above operation is applied.

     pixel level = (RAW pixel level)gamma_camera ~= exposuregamma_camera

The total system gamma is gamma_display * gamma_camera. Standard values of display gamma are 1.8 for older color spaces used in the Macintosh and 2.2 for color spaces used in Windows, such as sRGB (the default) and Adobe RGB (1998).

The three curves on the right, produced by Stepchart for the Canon EOS-10D, show how Gamma varies with RAW converter settings.In characteristic curves for film and paper, which use logarithmic scales (e.g., density (–log10(absorbed light) vs. log10(exposure)), gamma is the average slope of the transfer curve (excluding the “toe” and “shoulder” regions near the ends of the curve), i.e.,

Gamma is contrast.

See Kodak’s definition in Sensitometric and Image-Structure Data.

To obtain the correct MTF, Imatest must linearize the pixel levels— the camera’s gamma encoding must be removed. That is the purpose of Gamma in the SFR input dialog box, which defaults to 0.5, typical for digital cameras. It can, however, vary considerably with camera and RAW converter settings, most notably contrast.

Characteristic curves for the Canon EOS-10D with three RAW converter settings are shown on the right. Gamma deviates considerably from 0.5. Gamma = 0.679 could result in a 9% MTF50 error. For best accuracy we recommend measuring gamma using Colorcheck or Stepchart, which provides slightly more detailed results.

Confusion factor: Digital cameras rarely apply an exact gamma curve: A “tone reproduction curve” (an “S” curve) is often superposed on the gamma curve to extend dynamic range while maintaining visual contrast. This reduces contrast in highlights and (sometimes) deep shadows while maintaining or boosting it in middle tones. You can see it in curves 1 and 3, on the right. For this reason, “Linear response” (where no S-curves is applied on top of the gamma curve) is recommended for SFR measurements.

The transfer function may also be adaptive: camera gamma may be higher for low contrast scenes than for contrasty scenes. This can cause headaches with SFR measurements. But it’s not a bad idea generally; it’s quite similar to the development adjustments (N-1, N, N+1, etc.) in Ansel Adams’ zone system. For this reason it’s not a bad idea to place a Q-13 or Q-14 chart near the slanted edges.

To learn more about gamma, read Tonal quality and dynamic range in digital cameras and Monitor calibration.

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Electronic Imaging 2017


We are pleased to announce that we will return to the Bay Bridge, San Francisco at dusk. Shot from Yerba Buena Island.IS&T Electronic Imaging conference in 2017, to take place from January 29 – February 2 in Burlingame, CA. The conference will be held at the Hyatt Regency San Francisco Airport and will feature 20 technical conferences covering a variety of electronic imaging topics, ranging “from image sensing to display and hardcopy to machine vision in use in transportation systems” (Electronic Imaging). (more…)

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Testing a macro lens using Checkerboard and Micro Multi-slide

Testing a 1-5x Macro Canon MP‑E 65mm Lens

Imatest’s Checkerboard module is our new flagship module for automated analysis of sharpness, distortion and chromatic aberration from a checkerboard (AKA chessboard) pattern. The big benefit of using the checkerboard is that there are looser framing requirements than with other kinds of test targets. While checkerboard lacks the color and tone analysis provided by SFRplus and eSFR ISO, these features are not available on the high precision chrome on glass substrate, so the checkerboard is the optimal pattern for this test. (more…)

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Using Sharpness to Measure Your Autofocus Consistency

Autofocus plays a major role in many camera system applications with variable focus, including consumer electronic devices. Camera systems must be able to focus at a variety of distances. Optical systems on cameras only allow a certain range of distances from the camera to be in focus at once (this is often known as the depth of field, or depth of focus). (more…)

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Image Quality Testing for the Automotive Industry

Over the last few years, the automotive industry has taken great strides to incorporate camera systems into vehicles. From ADAS to rear-view cameras, there are many applications for camera systems in vehicles. 

With this integration of camera systems in the automotive industry, companies now have a variety of image quality factors to verify and standards to adhere to.  

Imatest is a leader in image quality testing solutions for the automotive industry. Join us for this free webinar to learn more about image quality testing in the automotive industry.

During this webinar, you can expect to learn:

  • An introduction to image quality factors and industry standards
  • What image quality factors matter most to the automotive industry
  • An overview of tests to deploy for automotive camera systems
  • How to begin testing your camera systems
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Fundamentals of Image Quality Testing

For many engineers tasked with designing camera systems for a product, image quality testing is a complicated endeavour given the variety of test charts and measurements available. 

Understanding which image quality factors to measure in your product and how to interpret results will ensure your product is equipped with quality imaging systems. (more…)

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Introducing Imatest 4.5

Imatest 4.5 is the most powerful version of the platform yet with enhancements and additions for a variety of industries, including automotive, medical, consumer electronics, security, and more. The newest features give users increased flexibility in the range of target distances and light levels to improve your high-speed, automated testing.

To learn more, please visit 

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The Operator Console: Open Source GUI for Imatest IT

Update: Starting in Imatest 4.5, a pre-built, ready-to-run version of the Operator Console now ships with all Imatest IT Windows installations, allowing users to take advantage of the simplicity of the Operator Console without writing a single line of code. It can be found in the /samples/OperatorConsole directory.


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Account Administration for Floating Licenses

Floating licenses can remove a lot of headache in keeping track of your licenses by allowing everyone on your team to use the same license. However, Imatest licenses require that the license code by assigned to one user account. This can be handled in one of two ways.


Assigning the license to one admin user


This is the easiest to implement method, and it’s how we have done it internally at Imatest. The user should not be the license purchaser, because the user’s store account will be used by the rest of the team.


  1. If the designated admin does not have an Imatest store account, they can create a new account on the Imatest customer portal.
  2. The admin user should then activate Imatest using their Imatest store account.
  3. The license is now “assigned” to that admin user that activated Imatest.
  4. The admin will share their account login details with all other users who will use that admin user account to activate Imatest on their machines.


If a company’s admin user changes for any reason, please contact us at, and we can quickly change to a different user that you designate.


Using a general licensing email account


This method requires the generation of a general email account unused by others, but it needs to be real in order for the email verification to work.


  1. Create an email account, like This email will be used for license support renewal notifications so the emails should get forwarded to the appropriate person.
  2. Use the generated email to create an Imatest store account via the Imatest customer portal.
  3. Have a user activate Imatest using the store account created in step 2.
  4. The license is now assigned to that store account with generic email.
  5. Provide the login details (email and password) for the general admin account to all users so they can activate Imatest on their specific machine.
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Imatest and Sofica Partner for Automated R&D Testing

Imatest is pleased to announce their partnership with Sofica, a group of camera algorithm validation and test automation experts based in Finland. This partnership will bring mobile device customers a revolutionary, robot-aided R&D test automation system integrated with Imatest analysis software and test charts.


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Beijing, China Training Course and Seminar


Imatest in Beijing, China

Imatest representatives will visit Beijing April 24-26, 2018 to offer a free Information Seminar and paid 2-day Training Course to professionals using or considering Imatest software to improve their image quality testing processes.

2-day Training Course

image (2)

The training course on April 25 & 26 offers attendees insight on the full capabilities of Imatest software in both research development and manufacturing environments. Training starts at 9:00 and will end at 17:00 – 18:00, depending on questions. Location is theNikko New Century Hotel.

View the detailed Training schedule.

Sign up online or contact a reseller in your area.

Free Information Seminar

If you are interested in finding out more about how Imatest software can improve your image quality testing, we encourage you to come to our free Information Seminar on April 24. 

Please register if you are interested in attending.

We look forward to meeting you in person.



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Karlsruhe, Germany Training Course and Seminar

Imatest in Karlsruhe, Germany

Imatest representatives will visit Karlsruhe, Germany the week of September 12th to offer a free Information Seminar and a paid 2-day Training Course to professionals using or considering Imatest software to improve their image quality testing processes.

2-day Training Course

The training course on September 15th & 16th offers attendees insight on the full capabilities of Imatest software in both research development and manufacturing environments. It will be held at the Novotel Hotel Karlsruhe. Training starts at 9 am and will end at 5 to 6 pm, depending on questions. Look for signs in the hotel lobby to direct you to the conference room.

View the detailed Training schedule. 

Sign up online or contact a reseller in your area.

Free Information Seminar

If you are interested in finding out more about how Imatest software can improve your image quality testing, we encourage you to come to the free Information Seminar before the 2-day Training Course. Seminar date and location TBA. Sponsored by Lensation.

Please register if you are interested in attending. 

We look forward to meeting you in person. 

Photo courtesy of KTG Karlsruhe Tourismus GmbH

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Imatest Support for CPIQ Metrics

What is CPIQ?

IEEE-SA working group P1858 created the CPIQ standard. CPIQ seeks to standardize image quality test metrics and methodologies across the mobile device industry, correlate objective test results with human perception, and combine this data into a meaningful consumer rating system.

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Imatest shuts down intermittently due to video driver problem


Imatest version 4.2+ intermittently halts.  A MATLAB crash dump file is generated in the user’s %temp% folder.

The crash dump file will start with text such as:

Assertion in void __cdecl `anonymous-namespace'::mwJavaAbort(void) 


An older version of NVIDIA video drivers has compatibility issues with the new graphics libraries introduced in Imatest 4.2 (MATLAB R2014b+)


Update your video drivers to the latest version.

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Imatest Version 4.4

Imatest is pleased to announce the release of version 4.4, which includes the following features:

Support for all 2016 Camera Phone Image Quality (CPIQ) standard metrics for mobile device image quality. CPIQ serves as a way to assess and communicate image quality to the vast majority of consumers who are unsure how to judge and compare device image quality. See a full CPIQ overview.

Auto-focus, Auto White Balance, and Auto Exposure (AAA) metrics for faster image adjustments. AAA metrics were created to address the video image quality aspects that most affect user experience.


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Imatest 4.3

We are pleased to announce Imatest 4.3. New features include:

    • Support for Linux (Ubuntu 14.04) across all Imatest software
    • Python API for IT for a simplified, flexible development environment
    • SSIM module for estimating the loss of image quality caused by image processing, especially data compression


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