I want to get brightness information from my iPhone camera.

https://qiita.com/koogawa/items/e0b3862c0fbdf97b579f
Can't I do this?

The screen brightness can be obtained as follows:

Objective-C:

CGFloat brightness=[UISscreen mainScreen].brightness;

Returns a value between 0.0 and 1.0.The more numbers you have, the brighter it becomes.

By the way, this is the case for Swift.
Swift:

var brightness:CGFloat=UISscreen.main.brightness

Screen brightness = surrounding brightness, so if the brightness is high, it is possible to judge that the user is in a bright place.
However, some users have fixed screen brightness, so this standard is not always true.

That's right.