Q: iphone 4s ios 8.1.2 auto brightness auto dim in bright place

Hello there hebe2003,

It sounds like your Ambient Light Sensor keeps automatically dimming the screen when you have that function turned on, no matter the light conditions. I would recommend these troubleshooting steps from the following article, named:

Get help with the screen on your iPhone, iPad, or iPod touch

 

1. If you have a screen protector, try removing it. Try removing your case if it includes a screen protector.
2. If you see the issue in multiple apps, restart your device.
3. If restarting doesn’t fix the issue, restore your device.
4. If you still see the issue after you restore, contact Apple Support.

Thank you for using Apple Support Communities.

Take care,

Sterling

Thanks for the update. Based on all the troubleshooting you've done this does appear to be a hardware issue, and would recommend reaching out to Apple to explore your service options with the links I provided previously.