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On Thu, 06 May 2010 00:56:35 +0200, Jim Henderson <nos### [at] nospam com>
wrote:
>
> I've actually wondered this myself - raw editing software gives you the
> option to adjust the exposure; obviously, you can't pull details out
> doing this that are completely washed out or completely underexposed, but
> it is possible to bring additional detail out by making changes to the
> exposure setting (ev) after the photo has been taken.
>
> I've done it, so clearly it's possible, I just don't understand the math
> behind it.
When the camera creates a JPEG file, part of the raw data is discarded and
the rest is then adjusted to fit in the 0-255 range. The camera picks a
black-point somewhere near the low end of the (raw) range, and all values
below that are made black (i.e. 0) in the JPEG. Similarly, a white-point
is selected, and all values above that are made white (i.e. 255). When you
edit a RAW file, you have access to the parts that would otherwise have
been cut off. If the exposure was somewhat less than perfect, there can be
useful data in those parts.
Also, the raw data has a higher bit-depth than a JPEG, typically 12 or 14
versus lossy compressed 8. This gives you more leeway for making
adjustments without getting posterization.
--
FE
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