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Old August 25th 07, 06:16 PM posted to rec.photo.digital
HEMI-Powered[_2_]
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Posts: 447
Default Compression in JPEG files in digital cameras

Bill Tuthill added these comments in the current discussion du
jour ...

HEMI, could you please post an impage where you can easily
see JPEG artifacts at the highest quality your Rebel
produces? The reason I ask is, it's hard for me to see JPEG
artifacts at Q 95 and 1x1 or 2x1 chroma subsampling.


Incidently, I don't think I ever said I've seen an artifact
in my Rebel's higher of its two available quality settings.
What I said was that some significant percentage at the lower
setting DID exhibit artifacts.


OK, thanks. This kind of supports my assertion (on another
thread) that the initial JPEG encoding imposes little lossage
on an image other than what the Bayer-pattern sensor has
already imposed.


My experience with now the 3rd of my digitals - 1st two were EVFs
- is that your assertion is right, IF you choose the best quality
at a given mega pixels, i.e., lowest compression.

As to Chroma subsampling, my normal setting is 1x2. I will
lower the compression number and/or change to 1x1 (none)
until I can eliminate the damage as described above.


It probably does no good, and might actually harm your image,
to switch from 2x1 chroma subsampling to 1x1. (I trust your
Rebel is writing 2x1, not 1x1.)

I'm told that my Rebel writes to 2x1, I was talking about what
/I/ choose when I am post-procesing in PSP 9. Each step down the
scale makes the file size smaller and presumeably the image a
little worse. I have found that 1x2 is a very good compromise for
me. You may find that 2x1 works fine. But, as I said, if I cannot
fix the artifacts by lowering compression, I will go to 1x1, that
almost always takes care of them.

--
HP, aka Jerry