Adaptive Brightness and Adaptive Contrast are two mechanisms by which the Surface Book's Laptops and Surface Pro change the brightness and contrast of the screen depending on the ambient conditions and what you are viewing.
Surface Book 2 can attach backwards to be easier to watch films in bed
Adaptive brightness in Windows 10 works similar to how your phone adjusts the brightness of the screen depending on the light it perceives. This prevents the screen from being brightly lit at all times and saves battery life when the computer isn't being charged.
Adaptive contrast auto-adjusts aspects of the screen based on light or dark scenes. This can give a richer experience to gaming and movies, however, it is very difficult to work with photos where you need accurate colour representation. It can also be annoying if you regularly switch between a "dark mode" application and a white, or light, web page as the screen ramps up the brightness.
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Disable Adaptive Brightness in Windows 10
You can easily enable or disable the adaptive brightness feature from the advanced options in your current power plan. To do that head to the Start Menu and search for "Power & Sleep settings". Open the link that appears.
In the related settings section, click the link for Additional Power Settings.
Disable Adaptive Brightness in Windows 10
In the window that pops up, click on Change Plan Settings next to the selected power plan.
Disable Adaptive Brightness in Windows 10
In this new window, find and click on the link for "Change advanced power settings".
Disable Adaptive Brightness in Windows 10
Now locate the Display options, then Enable adaptive brightness. From here you can turn the options off to disable Adaptive Brightness in Windows 10.
Disable Adaptive Brightness in Windows 10
Disable Adaptive Contrast in Windows 10
Unfortunately, there is no easy way to disable the adaptive contrast. While many users combine the two, disabling auto-brightness won't solve this behaviour if it bothers you. Microsoft does not offer any way to disable adaptive contrast through the Windows 10 interface, so the only way to do it is through the registry.
Warning: Modifying the system registry can have dire repercussions if not followed precisely, including driver failures, system instability, or not being able to boot into Windows. Users are always encouraged to run a system backup restore point before making any changes to the registry.
Launch RegEdit and navigate to Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4d36e968-e325-11ce-bfc1-08002be10318}\0001.
Scroll down to mid-page, and then find FeatureTestControl and double-click to edit the DWORD Value. Change Value data from 9240 (default) to 9250 (leave in base hexadecimal).
If the value does not exist in this location, instead navigate to Computer\HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4d36e968-e325-11ce-bfc1-08002be10318}\0000. There is a difference between Surface Books, Surface Pro and Windows update versions.
Hit OK to save the changes and restart the Surface to apply changes.
To re-enable the feature, just retrace the above steps and change 9250 back to 9240, save, and restart.
I've tried to disable adaptive contrast through the registry but I do not have the option of "FeatureTestControl" in either of the places suggested. I'm on a Surface Book.
It's possible that MS removed this option from the registry in a recent update. You could try creating the FeatureTestControl and see if that makes a difference. Unfortunately, FeatureTestControl is a pretty common setting name so you can't just search for it in the registry to find the contrast setting.
I just updated my Surface today and mine was still in the 0001 directory, so changing the hexadecimal for the FeatureTestControl worked. I'm not sure what the difference is here...