Question

The iPhone and iPad have different AvCaptureDevice methods to lock or put on Auto: white balance and exposure settings.

I am trying to understand the mechanism of the “Backside illumination sensor” on the back camera and now the front camera and whether it adjusts the white balance and exposure setting and if that is the case, would locking the WB and exposure mode would be interfering with the “Backside illumination sensor” job.

Or, is the “Backside illumination sensor” simply and only boosting the RGB values of the pixels in low light?

Thanks

Answer 1

I think you’re a little confused about what the term “backside illuminated sensor” means here. This is a just a type of CMOS sensor used in the new iPhones (and other mobile phones). It claims to have better low-light performance than older CMOS imagers, but it is just what captures the photos and videos, not a separate sensor for detecting light levels. There is a light sensor on the front face of the device, but that’s just for adjusting the brightness of the screen in response to lighting conditions.

In my experience, all automatic exposure and gain correction done by the iPhone is based on the average luminance of the scene captured by the camera. When I’ve done whole-image luminance averaging, I’ve found that the iPhone camera almost always maintains an average luminance of around 50%. This seems to indicate that it uses the image captured by the sensor to determine exposure and gain settings for the camera (and probably white balance leveling, too).