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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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It came from the Cathode Ray Tubes (CRTs) that were universally used for television and video display before modern flat screens became universal. In CRTs, screen brightness was proportional to the control grid voltage raised to the 2 to 2.5 power. For this reason, signals had to be encoded with the approximate inverse of this value, and this encoding stuck. As we describe above, in “What is gained by applying a gamma curve?”, there is a real advantage to gamma encoding in image files with a limited bit depth, especially 8-bit files, which only have 256 possible pixel levels (0-255).
Why are images converted from raw files often darker
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than JPEGs from the same capture?
Many cameras have the option of producing both interchangeable image files (almost always usually JPEG) and raw files from the same image capture. The raw-converted files are frequently darker— sometimes much darker— than the corresponding JPEGs. This is not an accident.
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Michelson contrast is named after Albert Michelson, whose work on the famous Michelson-Morley experiment led to was a critical factor in the acceptance of Einstein’s theory of special relativity.
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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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