Compressing images... loss?

This has been interesting discussion concerning compression.

I have several questions concerning jpeg compression.

Is there a difference in quality of jpeg images with different
levels of jpeg compression? What do you lose by using a lower level
jpeg compression to save the files? You do save disk space but do
you lose anything else?

I have a canon powershot and the options are superfine (very large
files), fine (large files), and normal. I guess the normal has the most
compression and the superfine has the least compression. Please
correct me if I'm wrong.

Thanks for any information. These little manuals are really don't
explain the differences.

Thanks

Alan

"Post Replies Here Please" wrote in message
...
This has been interesting discussion concerning compression.

I have several questions concerning jpeg compression.

Is there a difference in quality of jpeg images with different
levels of jpeg compression? What do you lose by using a lower level
jpeg compression to save the files? You do save disk space but do
you lose anything else?

It's a quality vs. size equation: the more compression, the less quality.

JPEG is a lossy compression that works by making various assumptions about
the way we see images and colors. For example, say you took a picture of
the sky - no clouds, just plain blue sky. If you zoomed down to the pixel
level, you'd find very small changes in the exact shade of blue from one
area to the next. JPEG compression assumes that you won't notice
differences of, say, 1% (this will vary with the compression level set), and
so defines anything that falls within that range as a single blue. This can
cause a "mottling" effect if you have a gradient where the color changes
gradually and you end up with "jumps" in color.

If you have one pixel of red within a region of blue, that may also be
discarded outright under the assumption that it wouldn't be noticeable or
missed. In some cases this may be desireable; in others it may have adverse
effects.

The higher the compression level you set, the broader and more forceful the
assumptions become... and the more noticeable their effect.

I have a canon powershot and the options are superfine (very large
files), fine (large files), and normal. I guess the normal has the most
compression and the superfine has the least compression. Please
correct me if I'm wrong.

Probably correct. Some cameras will use such definitions for different
image resolutions, and some may refer to a combination of resolution and
compression - one camera I used once, don't remember what kind, defined
highest quality as low compression at the maximum image size, mid-quality as
higher compression at maximum image size, and lowest quality at the same
compression as mid, but with a smaller image.

"Post Replies Here Please" wrote in message
...
This has been interesting discussion concerning compression.

I have several questions concerning jpeg compression.

Is there a difference in quality of jpeg images with different
levels of jpeg compression? What do you lose by using a lower level
jpeg compression to save the files? You do save disk space but do
you lose anything else?

It's a quality vs. size equation: the more compression, the less quality.

JPEG is a lossy compression that works by making various assumptions about
the way we see images and colors. For example, say you took a picture of
the sky - no clouds, just plain blue sky. If you zoomed down to the pixel
level, you'd find very small changes in the exact shade of blue from one
area to the next. JPEG compression assumes that you won't notice
differences of, say, 1% (this will vary with the compression level set), and
so defines anything that falls within that range as a single blue. This can
cause a "mottling" effect if you have a gradient where the color changes
gradually and you end up with "jumps" in color.

If you have one pixel of red within a region of blue, that may also be
discarded outright under the assumption that it wouldn't be noticeable or
missed. In some cases this may be desireable; in others it may have adverse
effects.

The higher the compression level you set, the broader and more forceful the
assumptions become... and the more noticeable their effect.

I have a canon powershot and the options are superfine (very large
files), fine (large files), and normal. I guess the normal has the most
compression and the superfine has the least compression. Please
correct me if I'm wrong.

Probably correct. Some cameras will use such definitions for different
image resolutions, and some may refer to a combination of resolution and
compression - one camera I used once, don't remember what kind, defined
highest quality as low compression at the maximum image size, mid-quality as
higher compression at maximum image size, and lowest quality at the same
compression as mid, but with a smaller image.