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The JPG image format is a great image format for reducing the size of the file.
Which means you can fit more images on a memory card, CD-ROM, hard drive or other media. And it is the
most widely supported format in the world. (meaning all photo programs will view it)
The downside with JPG images, is that every time a JPG image is saved,
it goes through a data compression algorithm.
This compression causes the resulting image to have slightly less quality than the original.
This "loss" is so small that you will not be able to detect the difference. Especially
if you save the JPG photo at 98% quality or above.
About the only way you will see a degrade or loss of quality with JPG, is if you save the photo
at a quality setting less than 95%.
Another way to lose quality is the "copying machine" affect:
Where you make a copy of a document and give someone the copy. (second generation)
They in turn, take the copy and make a copy of that for someone else. (third generation)
Who then makes a copy of their copy and gives it to someone else (fourth generation)
So on and so on.
Eventually, around the 8th generation copy, the document starts to degrade and become unreadable.
This can happen with JPG images also.
You save a JPG image at 100% quality, (2nd generation)
You then open that new file and save it again, (3rd generation);
and continue to do so until you are viewing the 5th or 6th generation; you will notice the colors of the image
have started to fade.
This is true with all JPG images regardless of the program you use to save the image.
What's the Solution?
With this information, you should be careful not to over edit/save, (more than a couple of times) your original images.
Another alternative, is to never overwrite your original JPG images.
Make copies of your originals and set the originals to be
READ-ONLY so they will never be overwritten.
Then, work with the copies when you want to edit.
Whenever you want to start fresh, go back to the originals.
Or, if you have enough hard drive space, convert your images to an uncompressed format like TIF or BMP format.
Which does not have this problem. Then, edit and save all you want. When you are done, convert them back to JPG.
Doing this will reduce the JPG saving to just one time, eliminating any noticeable loss.
Saving JPG images is not the same as copying JPG images.
When you save a JPG, it goes through the compression algorithm.
When you copy a file or copy a JPG image (using Copy/Paste commands) in PhotoELF or Windows Explorer, there
is NO degradation or loss of quality at all.
Copying a file or JPG image creates an exact duplicate of that file, bit for bit. It is identical and there is NO
quality loss at all.