Introduction – Encoding vs. Display gamma – Why logarithms?
How gamma-encoding increases dynamic range – Expected gamma values – Entering gamma into Imatest
Gamma and MTF – The effects of gamma errors on MTF
Which patches are used to calculate gamma? – Why gamma ≅ 2.2? – Why are raw-converted images often dark?
Tone mapping – Contrast definitions: Ratio, Weber, Michelson – Logarithmic color spaces – Monitor gamma
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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. We’re putting them together here in anticipation of questions about gamma and related concepts, which keep arising.
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log10(expmin)=−1/γ ; expmin=10−1/γ
For gamma = 0.5 (shown in the plot, above right), expmin = 10-2 = 0.01, equivalent to
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This brings us to the big question: Why is logarithmic encoding widely used in video/cinema systems? Coming from still photography, where gamma encoding is widely used (but log encoding isn’t), we are aware that gamma encoding accomplishes the same goals without the dynamic range limitation. If you have a good answer, please let us know. |
Misinformation warning — The Internet is full of misinformation about logarithmic color spaces. A good example is the B&H page, Understanding Log-Format Recording, which says, ” If your video—like most video—is not recorded using a log picture profile, chances are the exposure is being recorded in a linear fashion.” This is completely incorrect. The author seems to have no inkling that gamma encoding exists. Then he compounds the confusion with statements like “The problem arises with the realization that the exposure values with which we measure light are not linear. An exposure “stop” represents a doubling or halving of the actual light level, not an increase by some arbitrary linear value on a scale. You could, in fact, say exposure values are themselves logarithmic!”. Exposure Value (EV) is a relative measurement unit based on factors 2 (log2(exposure level)) that corresponds to the sensitivity of the human eye. It has nothing to do with the camera itself. For the record: Most digital sensors have a linear response, with more than 8 bits for high quality cameras (12, 14, or more bits are common). Following Analog-to-Digital conversion, most still cameras encode the image using a gamma curve. Video cameras use either gamma or log curves. As we mentioned above, this extends the effective dynamic range for gamma encoding. |
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