I think for Dolby Vision content the problem is even worse. Auto-brightness is entirely off track when watching such content. This is what brightness control is supposed to do during Dolby Vision is played back:
- 50% is the default value: it allows for the maximum brightness with a gamma curve as intended by the movie director
- Brightness settings below 50% reduce the HDR effect: it dims light scenes and thus makes the movie appear darker, but it also increases the the minimum black level, making dark scenes look more washed out and "LCD-alike" even on OLEDs.
- Brightness settings above 50% do not increase the actual brightness level of the content, but change the gamma curve, which basically means it increases the contrast in a controlled way and based on meta data. Dark scenes might look even darker at 100% brightness than at 50%.
Apple's implementation of the brightness control for Dolby Vision is absolutely correct on the one hand. On the other hand, however, the automatic brightness control blindly adjusts the brightness even when in Dolby Vision mode, leading to a continuously changing display mode for Dolby Vision. Auto brightness should be turned off automatically when in Dolby Vision mode, select a default setting of 50% and remember any deviation from 50% when manually set by the user for the next time Dolby Vision is being played back. This behavior is standard for OLED based Dolby Vision TV sets and should be being followed by Apple, too.