Anyone else notice Auto screen brightness kills battery faster than fixed?

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Even though I can't confirm it with Battery monitor I swear the auto brightness feature itself uses some battery.

Even when I'm in total darkness (where auto bright would be set low) it uses more battery than just setting it fixed at the same brightness.

I keep it fixed at 30% and get great battery life.

If you have had it on bright it will seem dim at first. But just after little while it seem plenty bright in all conditions. Except direct sun of course but nothing is bright enough for that any way.