In-camera exposure compensation (EC) vs. EC during RAW conversion

[atvinnys]

when shooting while bracketing, I definitely understand how exposure compensation works (basically double the exposure time for instance for 1 stop, etc..)
now, if you do this on a raw file, not sure I understand the whole SW thing.

1/ what does it do? is is a simple shift of the histogra?
2/ are there trade off applying exposure compensation in post processing. For instance: to avoid long exposure time and therefore noise, you would think you could significantly cut your exposure time with a SW that allows you to overexpose later on by 5 stops for instance.
now,besides the pain of correcting the exposure later on on post process, I got to believe that this is not the only drawback. lose of detail on under exposed picture would be first on my list.
In that case, how would you make sure you have enough details to correctly expose later on in SW?

Thanks
Vincent

[McQ]

Exposure compensation (EC) performed on the RAW file is done in software, whereas EC performed in the camera just changes the original exposure settings. In other words, in-camera EC changes how the image is captured instead of changing the image after it is captured. EC performed within the camera will therefore have no effect on image noise, clipping latitude or apparent dynamic range. By contrast, there are many drawbacks to software EC. EC performed in software on the RAW file can dramatically increase noise if it is positive EC (because the shadows can become the midtones); if it is in the negative direction then you will have a limited dynamic range in the highlights (these will be clipped much sooner than if EC is applied in camera).

Always perform EC within the camera itself if you have a choice. Applying EC to a RAW file is still better than adjusting the exposure of a TIFF or JPEG because it is performed on the untouched (high bit depth) pixel information from the camera's digital sensor.

[SRH]

Originally Posted by McQ

...EC performed in the camera just changes the original exposure settings. ... EC performed within the camera will therefore have no effect on image noise, ...

McQ, I agree with the principles you outline, but beware--my Oly E3 in "full auto" mode will adjust the ISO setting to achieve EC, and that, as you know, introduces noise. I don't know if other cameras do the same. I normally have my ISO set to a fixed value, so it doesn't really bother me--it's just something to watch for. The E3 has user settings to limit the range of auto ISO, and that is another way to limit the change in Auto ISO.

[McQ]

Thanks for clarifying your experience with EC, SRH. I was not aware of cameras that changed ISO in accordance with changes in EC. Makes sense though that in full auto mode, if the camera cannot achieve the right shutter speed, it might resort to changing ISO if the aperture cannot get any larger... My response should have had the caveat that that analysis primarily applies to SLR cameras.

[TonyBeach]

Originally Posted by McQ

...there are many drawbacks to software EC. EC performed in software on the RAW file can dramatically increase noise if it is positive EC (because the shadows can become the midtones); if it is in the negative direction then you will have a limited dynamic range in the highlights (these will be clipped much sooner than if EC is applied in camera).

Actually, beginning with an optimally exposed image (ETTR), applying negative EC via the RAW converter will decrease noise and increase DR. For Nikon cameras increasing EC in the RAW converter will have the same effect as raising ISO in the camera and can actually preserve highlights that might otherwise be clipped if ISO is raised in the camera.