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What is happening under the hood when a 5D Mk3 displays a histogram (luminance or RGB)? What exactly is the data it uses (raw sensor data, jpeg or something else)?

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The majority of cameras use the JPEG preview (with picture styles applied) embedded in a RAW file as the source data for the histogram and blinkies. In other words, it's just using what's sent to your LCD display for review. This is probably due to limited processor load and data paths, and is a cause for complaint among photographers--that the data they're seeing represented is based on after-compression data of roughly 1/10th of the image. You may actually have more latitude on either end of the scale with RAW files.

The closest you might be able to get to true representation of the histogram of your RAW file is to use the Magic Lantern firmware add-on and enable RAW histograms.

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  • \$\begingroup\$ Do you have a source for the claim that most cameras use the JPEG preview to generate the histogram? Your post makes it clear that this is plausible but it would be nice to have something more concrete. \$\endgroup\$
    – David Richerby
    Commented Feb 7, 2016 at 22:14
  • \$\begingroup\$ If you turn on blinkies and then compare to the raw later (i.e. In LR) you'll find the raw has A LOT of headroom left that is not visible in the preview. This is enough to convince me. \$\endgroup\$
    – Jim Garrison
    Commented Feb 8, 2016 at 1:38
  • \$\begingroup\$ @DavidRicherby The knowledge that in camera histograms are based on embedded jpeg previews is so well known that asking for documentation is about like asking someone for documentation that "a" is the first letter in the English alphabet. \$\endgroup\$
    – Michael C
    Commented Feb 8, 2016 at 2:57
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    \$\begingroup\$ @DavidRicherby: ronbigelow.com/articles/camera-histogram/camera-histogram.htm, ppmag.com/web-exclusives/2007/12/…, etc. lots of different internet threads dating back to 2005 :), and just comparing the histogram on the back of the camera to what you see in DPP or ACR from a RAW file. Most of us have experienced the additional headroom in RAW the camera histogram says isn't there. \$\endgroup\$
    – inkista
    Commented Feb 8, 2016 at 3:32
  • \$\begingroup\$ @MichaelClark This kind of statement is completely pointless if you don't back it up with sources. And being condescending isn't impressing anyone. \$\endgroup\$
    – Undistraction
    Commented Feb 8, 2016 at 7:13

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