On Aug 18, 1:55 pm, 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.

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.

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.