scott wrote:
>> ...but once the image has been taken, the exposure has already 
>> happened. How can you change it after the fact?
> 
> Think about how the camera works (in a very simple way).  It counts 
> photons for a certain length of time, applies a scale factor, then gamma 
> correction and writes the byte values to a JPEG file.
> 
> If you start with the JPEG file, you can undo the gamma, scale the data 
> using some exposure-adjustment factor, then reapply the gamma.  You then 
> have a new file that will look similar to if the camera had used a 
> different exposure.  Obviously the further your scale factor is from 
> 1.0, the more artifacts will be introduced to the image.
> 
> However pros use the raw sensor data from the camera and not a JPEG.  
> This allows them some margin to adjust the exposure later without adding 
> any artifacts to the final JPEG image they create.  Because of this it 
> is extremely important not to saturate the sensor (ie 100% white) in any 
> areas, it is impossible to get back detail in areas that are at 100%.  
> If you under-expose it you can scale up the brightness without 
> introducing artifacts (because usually the raw sensor data is higher bit 
> depth than JPEG).

...in other words, you're not changing the exposure (i.e., the number of 
seconds that the shutter opens) at all, you're simulating it.

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