If you start editing your original camera jpegs and save them on top of the original image you will lose image quality every time you do it.
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There are several other file formats available if you want to edit your images or send them by email or post them on the internet in a different format.
However, I would suggest you stick with the following which is adequate for most purposes.
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Download your camera files from your camera in jpeg format
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Save them all as Tiff images immediately without any modifications whatsoever.
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Never over write this file by saving on top of it.
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If you want to edit any of them - open the appropriate Tiff file and do your editing
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Save the image as a jpeg. This is the best overall format for emailing, uploading images to the internet, Ebay etc and for use in books, magazines etc.