Raw RGB (in DNG) and hidden exposure compensation

Question

ColorGrail OP

Created Nov ’16

Replies 2

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Hello,

I am currently working on the rear camera sensor of an iPhone and I have noticed a strange behavior with exposure settings and raw RGB data (in DNG). The same problem occurs with both iPhone 6S and iPhone 7 on iOS 10.0.1 and Xcode 8.

I made two experiments :

- In the first, I set manual shutter speed and manual ISO (with setExposureModeCustomWithDuration:ISO: ) and sometimes the luminosity of the picture changes (which should not occur as exposure parameters are fixed)

- In the second, I want to check the linearity of raw RGB according to ISO (exposure duration is blocked). At the begining everything is almost perfectly linear but when I increase ISO to a certain value, raw RGB do not change (though they are far from being saturated) while in photos the luminous shifts are present as expected. I have found out that this information is stored in the DNG EXIF data in the Baseline Exposure (BLE) tag. When I correct those RGB with this exposure shift (using 2^BLE, the formula that I believe to be correct), I lose the linearity between RGB and ISO, which is a big problem for me. In addition to this, the tone curve in DNG file is perfectly linear which I believe is wrong in the latter case.

I believe that the two problems are correlated and that in both cases the camera does a hidden exposure compensation. Is there a way to block this behavior and if not, will it be possible to know what is happening in the sensor?

Best regards,

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Answer 1

WoodyWazzo OP

Jan ’17

Dear ColorGrail,

1. To apply BLE properly, try using the formula SQRT(2^BLE). I believe the BLE reports a hardware gain (in EV units) applied to all pixels (R, G and B) straight off of the Image Sensor (Focal Plane Array) independent of the ISO value.

2. I too have noted the change in luminence when all relevant parameters are fixed. There is some sort of additional AGC going on that we the developers apparently aren't privvy too. I have also noted this with the 8MP cameras in iPhone 6 models running iOS 9. There is also (I believe) a white balance adjustment close to the hardware that the redGain greenGain blueGain read-back values do not account for, kicking in for dark pixels. This all appears to be happening in the 16-bit RAW pre-gamma domain. This is just what I've observed...maddening. Apple sure doesn't make it easy to get linear photometric data from the camera!

-WW

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Answer 2

hmirzaei OP

Sep ’21

Hello guys,

Any luck here?

I am also working with raw DNG files I have captured with iphone 11 and iphone 12 ( I used Moment app to capture raw - not proraw). I want to check the linearity of raw RGB according to ISO. It is (almost) linear up to ISO of around 200. Apparently there is a Baseline Exposure and also a Exposure Bias (or Exposure Compensation) tag available in the DNG.

I correct the linear values based on 2^(baselineExposure+exposureCompensation) and it still does not fix my linearity issue.

The best fit (very strange) I could get to my data was 2^baselineExposure * 2^(-10*exposureCompensation).

Anybody knows how to interpret those tags?

Thanks, Hamid

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