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I'm using a 2011 13" MacBook Pro. For the past few days the brightness always resets to full on boot up and just when the spinning wheel stops loading (before desktop loads), it dims automatically to the level I've set it at.

The brightness level is sometimes maintained during boot up but its very sporadic.

I've already tried resetting SMC and PRAM but neither seems to have helped.

I thought my new Mountain Lion install had something to do with it so I reinstalled Snow Leopard but the issue still persists.

What could possibly be the issue? Any help will be appreciated.

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2 Answers

I've also noticed that my Macbook Pro boot sequence only occasionally ups the brightness to full when rebooting. (However, I reboot so rarely that I don't really mind.) I thought it was just me! I'm running OSX Lion (10.7.4), btw.

I like the suggested answer that the user preference value isn't read soon enough, but I suspect that the issue is that the appropriate driver isn't loaded until the last moment. From what I understand, the user prefs would be readable at any time during the boot sequence (assuming the root/boot disk volume is mounted, anyway), but if the driver isn't loaded, the value wouldn't be applied. That would account for the display being really bright for some time, but it doesn't explain the occasional (for me, about every second bootup) dimmer value.

Of course, if the dimmer code is part of the BIOS, that's completely wrong, and it probably would be down to when the user prefs were read and fed to the driver!

The strange unpredictable behaviour may depend on what sequence of events occurs when shutting down; it's possible (though not probable) that in some situations, the brightness register isn't reset when shutting down. This probably wouldn't happen if the machine was off for some hours/days/weeks. It would be interesting to see if there's a pattern to the behaviour though!

It might be possible to re-order the sequence so that the display brightness setting is applied sooner rather than later, but it will still be some number of clock cycles between the LED backlight being turned on and the appropriate driver load/preference read.

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This appears to be just how it works, unfortunately. The best solution I can come up with is to not ever shut your Mac down so you don't have to deal with an over-bright screen on bootup.

My Mountain Lion (previously Lion) MacBook Air does this too: full brightness on the login screen, then it dims after login. The cause is probably that the brightness preference is stored in the user account's settings, which can't be read until the user logs in.

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Thanks for the quick response. –  trmnl Aug 8 '12 at 17:42
    
I understand your point about the brightness preference being stored in user account settings but the brightness level doesn't always reset to full upon boot up. Sometimes it starts up correctly at the level i've set it at; which is still odd considering your point. –  trmnl Aug 8 '12 at 17:53
    
It doesn't always do that? Odd. My MacBook Air always turns the brightness up to max at the login screen. –  CajunLuke Aug 8 '12 at 18:22
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