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When I edit my pictures I always work in 16 bit mode because I know I have more color space where I can move so I can avoid issues like banding.

At the same time I realised that there's an advanced option to enable 30 bit display (so 10 bit per channel) for OS + monitors that support it.

Even considering monitors that have 8 bit per channel, I don't get how's possible to work in 16 bit mode. I miss the link between these two things.

Extra question: When I see there are monitors like Eizo that have 16 bit depth, how do they work if the operating Photoshop doesn't support it?

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When you work in 16 bits mode, the pixel data have 16 bits/channel, thus 65536 shades per channel.

Your display has 8 or 10 bits/channel, thus 256 or 1024 shades per channel.

The way you convert the source data to the display is by resizing the color space. You have different strategies called render intent.

The more simple, the perceptive intent, is a proportionnal resizing :

  • the 0 value of the source data is matched with the 0 value of the display,
  • the 65535 value of the source is matched with the 255 value of the display.
  • the n value of the source is matched with the n × (254 - 0) / (65535 - 0) of the display.

This is assuming that the color profile of the picture (let's say sRGB) is the same as the display. If not, you have to adjust the gamut in addition (dealing with the color values that the display will never be able to reproduce physically) and the computation is not that simple.

If the display (Eizo 16 bits) has more shades than the source image, Photoshop probably sends 8 or 10 bits data and then, on the operating system or on the screen itself, some algorithm resize them to 16 bits by doing the opposite operation : the n value of the source is matched with the n × (65535 - 0) / (254 - 0) value of the display.

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    \$\begingroup\$ Very clear explanation, thank you! It's more or less what happen sending a file to the printer I guess. \$\endgroup\$
    – Luigi Trevisi
    Commented Apr 26, 2017 at 22:06
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    \$\begingroup\$ How's possible then to see the difference between a picture worked in 16 bit or 8 bit mode? \$\endgroup\$
    – Luigi Trevisi
    Commented Apr 26, 2017 at 22:08
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    \$\begingroup\$ @LuigiTrevisi you can't. The bit depth makes a difference only when you increase dramatically the exposure or the saturation of some part of the image : having more bit depth makes the gradients more continuous and less prone to banding/posterization effect. An higher bit depth just means a wider range of post-processing adjustments. \$\endgroup\$
    – Aurélien Pierre
    Commented Apr 26, 2017 at 22:22
  • \$\begingroup\$ @LuigiTrevisi It's just like when you're working with a raw file. What you see on the monitor is not the actual raw file, it's an 8-bit (or 10-bit if your raw processing application, monitor, and GPU support that) conversion of the raw data as processed by the current settings of the raw conversion application and your computer's graphics settings. \$\endgroup\$
    – Michael C
    Commented Apr 27, 2017 at 1:32

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