A bunch of users have taken to Reddit complaining about poor low-light camera performance with Live Photos as well as some overheating issues preventing the activation of the camera's flash feature on their iPhone 6s and iPhone 6s Plus. The low-light camera performance issue is attributed to lack of enough exposure and limited shutter speed while taking Live Photos.
According to iDownloadBlog, the iPhone 6s records 1.5 seconds of video before and after the still photo is taken, thereby combining together small bursts of motion and sound using the iPhone's video capture mode. In other words, a Live Photo features a three-second video being captured at 14 frames per second, while the shutter speed of still photo simultaneously downscales to the lowest frame rate possible to match up with the video frame rate.
Consequently, a 12 megapixel image gets downscaled with an effective shutter speed of 1/14th of a second, which allows just a fraction of the light to pass through as the Live Photo mode will continue recording video in the background. The shutter speed limitations will result in blurrier than normal photos due to poor exposure and even the slightest hand movements.
Given the shutter speed constraint, the exposure has little time to adjust to the low-light conditions while snapping a still photo along with background video recording in Live Photos mode, as it has to adjust exposure twice in under one-thirteenth of a second. You can however disable the Live Photos feature to promote longer shutter speeds and improve the overall quality of still images being captured in low-light conditions.
In related news, some iPhone users have also been complaining via Reddit about some overheating issues preventing them from using or activating the camera's flash, until the phone has cooled down significantly. It is further ascertained that Apple's new A9 processor chip implemented in the iPhone 6s models is considerably faster than the A8 processor chip that powers last year's iPhone 6. As a result, the iPhone 6s models tend to overheat a lot more than their predecessors in order to offer the extra computational power in a small footprint.
Given the iPhone 6s's record-breaking performance benchmark results, it is no surprise that its internals are running a little hotter than expected, especially when the phone seems to shut down its camera's flash feature to prevent hardware damage due to overheating.