I must soon discuss photo with friends and I have a rather technical question:
How does exposure compensation works on a RAW file?
Is it a displacement of the gamma curve in the dynamic zone of the sensor?
If not, is this correction done before or after applying the gamma curve?
Thank you for your reply…
I’m not sure I understand :
When I take a photo and depending on the iris and speed settings my sensor has loaded its photosites with some photons.
The choice of ISO sensitivity somehow multiplies the raw value from the sensor by the value of the ISO (IL)
At the end, the gamma curve is then applied to correct the linear function of the sensor and adapt it to the sensitivity of the eye.
That’s how I think I know about the RAW treatment in my device. (I’m not very sur that is all correct?)
As you tell me, the exposure correction (in the RAW development software) would be a kind of simple adjustment of the ISO?
Exposure correction is usually done before gamma-adjustment (while the pixels have values that are proportional to object luminance). Double the values to give one extra stop, etc.
If the gamma adjustment is a simple formula, v’ = v^p, it can be done after gamma, by multiplying by a different number.
When you set exposure compensation in your camera, your camera manipulates one or more parameters to reach the target. For example if your camera meters that the best values to best expose the scene are ISO=100, f=8, t=1/120, and you want the photo to be 1 stop brighter, you set EC=+1, and then your camera will change one of those parameters to make the photo 1 stop more exposed. Which parameter changes depends on which camera mode you’re in. If you’re in aperture-priority mode, that means the camera is not allowed to change aperture, so it may halve the shutter speed from t=1/120 to t/60. In this example, making the photo 1 stop brighter did not increase the noise, it just possibly increased temporal blur. This means that the information captured changes and the contents of the raw file change.
When you set exposure compensation in post-processing software, it scales the numbers read from the raw file. That results in noise becoming more visible.