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Old August 18th 07, 08:33 PM posted to,
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Default Compression in JPEG files in digital cameras

wrote in message
I need some help in explaining the JPEG compression feature in digital
cameras. My camera (which likely is similar to most others) has the
feature to compress the photo JPEG files in the storage card. It also
has the choice to have different pixel sizes (example: 3000x2250,
2000x1500, 1024x768, etc). What is the difference of the above two
features? If you store a 3000x2250 pixel data in compressed mode,
does it loose its quality? Can it be re-instated to full uncompressed
size without loosing photo quality?. When I compressed the data, it
will fit more pictures in a single storage card. But, is it the same
if I choose 2000x1500 pixel and no compression instead?
Thanks for info.

All jpegs have some compression. In simple terms, because I am, jpeg saving
examines the image in 16 pixel groups and assess how much alike the colours
are. Just how much the need to be is the compression factor high compression
means they dont have to be as close as low compression. What it does now it
save one pixel of that colour and notes where the simler pixels are then
dumps them.
When you reopen the image the missing pixels are replaced with copies of
that saved one.

Images with large expenses of simler colour like sky and sand compress down
to a smaller file size at the same level of compression than a image busy in
fine detail. This happens each time you save as jpeg, losing some detail
each time until the image is ruined so dont keep making changes and resaving
none of this Makes any difference the dimensions such as 3000 x 2000 adding
the compression and smaller dimensions will make a mush smaller file. When
it comes to printing the more pixels the better if you want a larger print
as we use a print scaling factor referred to as ppi pixels per inch. That
how many pixels we pack into a SQ inch of paper. for a print that we will
inspect at upto arms length we need 200ppi. this means the finest detail in
the print will be 200th of an inch. so for a 10 x 8 print we need 10 x 200
= 2000 pixels wide by 8 x 200 = 1600 pixels high.
I hope that is of some help