Why when playing a movie on an iPad standard video player changes brightness automatically? How can I turn this off?

Hello there, Michael770.

It sounds like you are wanting to adjust the auto-brightness feature on your iPad. The following Knowledge Base article provides information on where that setting is:

Adjust the display settings on your iPhone, iPad, or iPod touch

You can adjust the display brightness manually, or Auto-Brightness can adjust it for you. Night Shift adjusts the color of your display after sunset and True Tone display adjusts the color of the iPad Pro display according to the ambient light.

Set brightness or use Auto-Brightness

• To change your screen brightness, go to Settings > Display & Brightness and drag the slider to the right or left.
• If your iOS device has an ambient-light sensor, you’ll see an Auto-Brightness setting under the slider. Auto-Brightness uses your device’s light sensor to pick the right screen brightness based on your surroundings. Auto-Brightness can improve battery life in some situations.

To personalize Auto-Brightness:

1. Go to Settings > Display & Brightness and turn on Auto-Brightness. You might notice that the brightness slider moves according to the current lighting conditions.
2. Adjust the slider to set the Auto-Brightness yourself—and Auto-Brightness will pick future brightness settings based on that.

If Auto-Brightness isn’t working as expected, you can reset it. Go to Settings > Display & Brightness and turn off Auto-Brightness. Then turn it on again.

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Cheers.