Hi all,

First off, I suggest file sizes of around 150 to 300KB. This is large
enough to provide good display, and small enough that to be affordable.

That being said, there seems to be some confusion about image size and
file size. Trying to reduce the image size in order to get a smaller
file sometimes degrades the image so much that you can see the jaggies
even when the image is not full screen.

The following may help:

1) Image file size depends on two things:
a) pixel dimensions: the more pixels, the larger the file;
b) colour depth: 24-bit requires 3 bytes per pixel. This is the most
common, but some cameras and many scanners store more colour information
per pixel. (footnote)

Thus a 1200x800 pixel image in 24-bit colour will have a file size of
1200 x 800 x 3 = 2 880 000 bytes. A bitmap file will be about 2.5MB in
size. This is quite large, so various compression schemes have been
developed. These all rely on the fact that any image file will have many
bytes storing exactly the same colour information, for example the black
background in Maroochy's lovely studies. Storing information _about_
those repeated bytes, instead of the bytes themselves, can result in
huge savings in overall file size.

2) The most commonly used scheme is JPEG. The size of the JPEG file
depends on
a) the compression ratio and
b) the amount of detail in the image.

Typically, a JPEG file will be about 10-15% the size of the bitmap file.
Our example file would be about 250KB.

The problem nowadays is that recent cameras all take images in the
10-20MB pixel range. Even the cheapie point'n'shoots are 10MB pixels or
more. This means very large image files. My camera (now 2 years old, so
already obsolete ;-) ) stores its 12MB pixel images as JPEG files of 3.5
to 4.5MB. The bitmap (RAW) files would be 36MB!

In most image-processing programs, you can set the JPEG compression
ratio, but the default is fine for most cases. I advise experimenting
with your image processor, and checking the file sizes. I suggest that
files should be around 200KB for this news group. I've found that
reducing my images to 1200x1600 results in JPEG files of around this size.

Please note that the display-size of the image on the screen is a whole
'nother issue. Most programs will automatically change the _display
size_ to fit the window or the screen.

HTH
Wolf K.

Footnote: because of physics, scanners have a very snall contrast range,
so the range is expanded by software. This takes extra image
information, so the better scanners these days use 42 bits (a little
over 5 bytes) per pixel. Thought you might like to know. ;-)