Don Stauffer in Minnesota added these comments in the current
discussion du jour ...

This is a complicated question, and not suited to quick
answers- I'd get a decent book on digital photography.
Actually, the Dummies Guide series has a reasonable book on
the subject.

For this OP, it may be quite simple. It's fine if the OP wants to
buy a book, probably a real good idea to learn the gentle art of
digital photography. However, few books can adequately illustrate
the effects of excessive compression, and a half-hour shooting a
few dozen test shots around the house and yard can be very
instructive to a new user. After all, it is THEIR definition of
"good" and "bad" that matters, not all of us would-be "experts."

Briefly, there are two different ways of reducing the size of
the file (memory size) of an image. One method is to
"downsample", or average pixels to create a new array of
numbers with fewer pixels. All the pixels in the image chip
are used, but the downsampling or averaging math reduces the
"resolution" or sharpness of the image.

Data compression schemes such as JPEG do not downsample in the
same way. Depending on the subject a jpeg will retain all or
most of the resolution. However, subtleties of color
disappear, gradients of color or brightness get flattened, and
little regions of the wrong color begin cropping up. Jpeg is
a lossy compression scheme, meaning this loss of image quality
cannot be reversed after the image file is compressed. There
ARE other compression schemes that ARE reversible.

With the cheapness of very large memory cards these days, and
also large memory banks for computers, there is little reason
to downsample in the camera, or to use excessive values of
compression (many cameras allow you to select how much
compression to use). The most common advice these days is to
shoot full resolution (the 3000 x 2250 mentioned), and select
the least amount of jpeg compression. This is frequently
called picture quality, as in super high quality, high
quality, medium, or some such combination of words.






--
HP, aka Jerry