Peter Jason wrote:
I have Win7 SP1. I take pictures of about 4 MB with a good quality
camera. These are excellent, but I lose definition when these are
reduced to about 500KB with software such as Photoshop. When I take
small pictures with the camera the quality/definition is very good. I
don't want to take two shots of everything just to be able to get
small pictures for emailing. Why cant large quality pictures be
reduced to smaller ones - with the same definition?
Peter

There are a couple ways to shrink picture size.

1) Resolution change. You can use "scale image"
in your image editor, and change the locked together
"X resolution" and "Y resolution" settings from say
1200 DPI to 600 DPI. That cuts the number of pixels by
a factor of 4.

2) You can use a lossless compression format. I think GIF is
lossless, but it only supports pictures with 256 colors.
There are other examples here of lossless methods. You
might get a factor of 3 using a lossless method. Using
(1) and (2), we can get close to a factor of 12.

http://en.wikipedia.org/wiki/Image_compression

3) Once you move to a lossy compression method, then you're
in control of the picture quality versus size tradeoff.

http://en.wikipedia.org/wiki/Jpeg

If you look at the "Jpeg tombstone" image further down
that page, and look at Q=10, you get a compression of 46:1 .
Which is a significant improvement.

There are other methods of compression. The JPEG article
mentions the algorithm isn't applied all that intelligently,
and there are some options for further improvement. But then,
the person receiving the picture, might not have a decoder for it.

Another form of compression, involves fractals.

http://en.wikipedia.org/wiki/Fractal_compression

That article says fractal compression starts to win, at
compression ratios higher than 50:1. And that's why I
picked the Q=10 picture in particular, from the JPEG article.
If the picture quality at Q=10 is good enough, you probably
can't do much better with the fractal method. But if the
quality is unacceptable, then a fractal method taking a
long time to compress, may achieve a more pleasing result.

Most good lossy compression methods (100:1 compression), will tend
to have a weakness for certain kinds of content. They may do
better with "natural images", than with artificially generated
images (like a cartoon with solid colors). A cartoon sees
excellent compression with GIF, probably higher than the
factor of three that I quoted above.

The best way to learn about this stuff, is to play with it.

Paul