AVFoundation AutoExposure - camera

Question

I'm developing camera app right now.
I finished most of important features but the most important feature left.
I want my app auto exposure mode.
Here is my sample.
do {
try camera.lockForConfiguration()
camera.focusMode = .continuousAutoFocus
camera.exposureMode = .continuousAutoExposure
camera.unlockForConfiguration()
} catch {
return
}
I made this setting between lockForConfiguration and unlockForConfiguration.
This lead to autoexposure, however it is different from native camera app.
Also, shutter speed does not go below 1/40.
It is always 1/40 whenever it is in dark environment.(where native camera app adjust shutter speed to 1/2 or even 1)
I found the answer here (https://developer.apple.com/forums/thread/26227)
But I don't understand this answer and even how to do that...
session.beginConfiguration()
defer {
session.commitConfiguration()
}
device = AVCaptureDevice.default(.builtInDualWideCamera, for: .video, position: .back)
guard let camera = device else {
set(error: .cameraUnavailable)
status = .failed
return
}
I set AVCapturedevice in this code. Also there is a session.
The answer says don't touch sessionpreset, so I did do anything by sessionPreset.
But how can I set the avcapturedevice by activeformat..?
This is very hard to understand.
Just to check if this is work, I added this codes just below above codes.
'''
var bestFormat: AVCaptureDevice.Format?
bestFormat = device!.formats[2]
print(self.device?.formats)
do {
try camera.lockForConfiguration()
camera.focusMode = .continuousAutoFocus
camera.exposureMode = .continuousAutoExposure
camera.activeFormat = bestFormat!
// camera.setExposureModeCustom(duration: CMTime(value: 1, timescale: 500), iso: 2000)
camera.unlockForConfiguration()
} catch {
return
}
'''
I believe this does not work because camera-resolution doesn't change.
even if device!.formats[2] is low resolution for me (I checked it)