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For example, if I want to change camera ISO with the Pi Camera Module 2, I don't see any difference in images between:

camera.iso = 100
camera.exposure_mode = 'auto'
camera.capture(filename)

and

camera.iso = 800
camera.exposure_mode = 'auto'
camera.capture(filename)

which leads me to believe that the "auto" setting resets any of my ISO settings.

However if I make the same change, swapping in the ISO line for camera.brightness = 0 or camera.brightness = 100

... the difference is night and day :)

Any way to figure out what exposure_mode = "auto" actually dictates and what it doesn't?

Camera documentation for reference - https://www.raspberrypi.org/documentation/raspbian/applications/camera.md

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"Exposure mode 'off' is special: this disables the camera’s automatic gain control, fixing the values of digital_gain and analog_gain.

Please note that these properties are not directly settable (although they can be influenced by setting iso prior to fixing the gains), and default to low values when the camera is first initialized. Therefore it is important to let them settle on higher values before disabling automatic gain control otherwise all frames captured will appear black." - PiCamera docs

The example above just needs to give the camera time to change its gains.

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