Re: D200 vs D300 at ISO 1600

Floyd L. Davidson <floyd@xxxxxxxxxx> wrote:
Chris Malcolm <cam@xxxxxxxxxxxxxxxxx> wrote:

While jpeg compression is a lossy form of compression, in the sense
that you can't undo it to return to an exact replica of the original,
something is lost in the compression, non-linear encodings aren't
necessarily lossy compressions.

The added quantization distortion in the higher levels
is due to an error that cannot be subtracted after the
fact. The original signal cannot be recovered, therefore
whatever "compression" exists is by definition lossy.

You may be thinking of trying to recover the original analog
signal. Strictly speaking it's impossible to do that perfectly. It's
only possible to do it within defined limits of accuracy. Once you've
done it to the extent that your senses or your instruments can't tell
the difference it's as perfect as it needs to be.

There is a much stricter and simpler definition of lossy compression,
however, which applies in the case of jpeg compression, and that is
when we're talking about compressing a digital representation. In that
case exact recovery has a precise and very easily measured meaning. A
method of compression is lossy if you can't recover an exact replica
of the original file. Jpeg was designed as an inherently lossy method
of compression. If you translate another kind of image file to jpeg
and back again you inevitably lose something.

But there are lossless methods of file compression with which recovery
of an exact replica from the compressed file is possible. ZIP is a
well known example. In the field of digital image compression the new
JPEG 2000 and JPEG XR standards both include lossless compression
options.

--
Chris Malcolm cam@xxxxxxxxxxxxxxxxxxxx DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

.

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