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Introduction
In this post we discuss a number of concepts related to tonal response and gamma that are scattered around the Imatest website, making them hard to find.
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They are assembled here in anticipation of questions about gamma and related concepts, which keep arising.
Gamma (γ) is the average slope of the function that relates the logarithm of pixel levels (in an image file) to the logarithm of exposure (in the scene).
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There are actually two gammas:
encoding gamma, which relates scene luminance to image file pixel levels, and
display gamma, which relates image file pixel levels to display luminance. It is typically found in equations of the form,
Eazy math inline body \text{log(display brightness)} \approx \gamma \ \text{log(pixel level)}
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The chart on the right is designed for a visual measurement of display gamma. But it rarely displays correctly in web browsers (and may never display correctly on this Atlassian page, where there is no way to set it to 100% size). It has to be displayed in the monitor’s native resolution, 1 monitor pixel to 1 image pixel. Unfortunately, operating system scaling settings and browser magnifications can make it difficult.  |
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