ray wrote:
On Sat, 18 Aug 2007 11:55:45 -0700, aniramca wrote:

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.

It will uncompress to the full resolution - however it will not be an
exact replica of the original image. JPEG is a 'lossy' compression
algorithm - meaning that some detail is lost in the act of compressing the
data. There are 'lossless' compression schemes as well, but the level of
compression with them is generally considerably less. From a practical
standpoint, you can do a lot of compression on a JPEG image before you see
noticeable loss of information. Before I got a camera capable of saving
RAW images, I always chose the highest resolution and the smallest amount
of compressin on JPEG images - now I simply save nearly everything in RAW
- even though it does take a lot of space. Memory cards are very cheap now
- IMHO it is better to get some extra cards and save everything using the
best method possible. I currently have 2-1gb cards and 1-2gb card for my
5mp camera - they run around $15 and $30 respectively.


Currently several places in my local area sell 2GB SD cards for under
$20. 4GB cards go for about $40. Very little excuse to opt for more
compression, or lower resolution, just to save space. Note that if you
have a need for fast shooting, writing very large files (RAW/TIFF), may
slow down your shot rate, considerably.