powershot A630

the camera sensor is 8Mp
the pictures i make with this camera have around 2 Mb jpg format.
when i digitally zomm the picture in photodraw i can get up to 200% and stil have a good quality. Lets say i change something minor, like increase the contrast with 1%, keeping the dimensions and the rest the same. I save the picture in the same jpg format and has only arround 400 Kb ( the changed picture takes 5 times smaller space.) If I digitally zoom in photodraw the new smaller picture,
then up to 200% good quality, practically no difference related to original.
????
What is that big initial size for?
Does it take real 8Mp pictures?
How comes everything looks the same in a picture 5 times smaller (size in bytes).

ty

That sounds right. The JPG format is compressing it down to the 2MB file you're seeing. When you software opens the jpeg file it restores it to 8MB. When you save it, it's recompressed. Keep in mind that saving it repeatedly in jpeg format can cause the quality to go down. If your going to do a lot of processing, save it as a tiff or PSD (photoshop) file while you're working on it. Tiff files offer compression but it is lossless (no quality loss) compared to jpeg (which does throw data away).

Probably the reason the file got smaller when you saved it is because you can set the quality of the jpg when you save it. The lower the quality, the smaller the file (again you're throwing more data away).

As for scaling a photo up, you always loose quality. The best way I've found is to scale it up by 10% at a time until I reach the final size I need, then sharpen it. I've gotten 30x20 prints from a 5 MP camera this way and the print looked fine.

If you look at the new, smaller, file in an area where there's a large area that's almost the same colour or tone (like the sky), you'll probably notice "banding", areas where the colour "stair-steps" and changes abruptly rather than smoothly like it did in real life. JPEg uses "lossy" compression. It assumes that you won't notice the difference in small colour shifts so instead of saving each individual colour level it "averages" close-by colours and stores that average. The more compression you use, the broader range they compress these colours to. So if you're editing a photo, don't save it as JPEG; use either TIFF or the editing program's native file format until you're done making changes.

the problem is that the file at the begining, from the camera, is already in jpg format. it's not like going from bmp to jpg, but from jpg to jpg. for what you say it's like one kind of jpg in camera and another kind of jpg in programs.
the program opens a jpg file, decomprime it ( i hope the term is correct) to get the real thing, you do something like increase the contrast with 1% which shouldn't change the size of the file to much, then at exit the program re-comprime it in jpg. the difference between input and output is 5 times !!! for the same file format? jpg should be the same thing for any program because it's a standard
I will try the next thing:
i will do the same procedure open, increase contrast or tint by 1%, then save,
several times, to see how the size of the file changes.

ty

JPEG uses what's called a "lossy" compression. The degree of compression, hence how much data's lost, is set within the individual program. Take a look at the Save options in PhotoDraw. The greater the degree of compression you allow, the more banding/stair-stepping you'll get. And this is cumulative; if you open, edit, and resave a file repeatedly it'll continue to shrink and lose data. The JPEG format is a standard. But that format includes provisions for changing the degree of file compression when you resave. If you don't want to lose data when saving, save your files as TIFF or BMP or other lossless format.