screen keeps dimming on photos app

Hello~ What is your battery health? Go to Settings  > Battery, then tap Battery Health & Charging. Also are you using Low Power Mode?

Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support

If your battery is 80% or below then…replace it. Click on on below…

Genius Bar Reservation and Apple Support Options - Apple

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~Katana-San~

Hello!

What is the Maximum Capacity of your device’s battery? To find it, go to: Settings > Battery > Battery Health & Charging. 

Is the device warmer than usual when this happens?

Looking forward to your reply!