Night mode is an automatic setting which takes advantage of the new wide-angle camera that's in the iPhone 11 and 11 Pro models. It's equipped with a larger sensor that is able to let in more light, allowing for brighter photos when the light is low.
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Apple Releases iOS 12.4.2 Update for Older iPhones and iPads
Apple is making iOS 12.4.2 available for the original iPad Air, the iPad mini 2, the iPad mini 3, the iPhone 5s, iPhone 6, iPhone 6 Plus, and sixth-generation iPod touch, all of which are not compatible with iOS 13.
The iOS 12.4.2 update can be downloaded on eligible devices over the air by opening up the Settings app, selecting the General section, and tapping on Software Update.
It's not clear what's included in iOS 12.4.2, but it is likely a security update that adds important security fixes for older devices that weren't able to get those updates through the newer software. Apple's release notes say the update "includes improvements, provides important security updates, and is recommended for all users."
A security support document provided alongside the update indicates that it fixes a vulnerability that could allow a remote attacker to case unexpected application termination or arbitrary code execution.