Apple is separating the new smartphones into its usual low-cost versus high-cost categories, with big differences between the two models coming down to the camera, display, and battery life.
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Apple Revises 2018 MacBook Air Display Brightness to Up to 400 Nits After macOS 10.14.4 Release
We've confirmed with Apple that this increase is tied to the macOS 10.14.4 software update released on March 25. Apple's release notes did mention that the update "corrects the default screen brightness" for 2018 MacBook Air models, although no specific details were known at the time.
Prior to macOS 10.14.4, Laptop Mag found that its 2018 MacBook Air review unit maxed out at a brightness of 234 nits, while NotebookCheck determined an average value of 315 nits. It's unclear why there is such a big discrepancy there, but it'll be interesting to see how much the number increases in subsequent testing.
Battery life stats for the 2018 MacBook Air remain unchanged on Apple's tech specs page.