One benefit of shooting RAW image is that RAW photo editors do not change the RAW image data but only save or embed the editing information along with the image data. When the edited RAW photo is opened in the editing software later, the software re-renders the image using the original image data and the stored editing information. As a result you can mess with a RAW photo and go back to the original without any loss of image information. This is not the case for JPEG photos. Each time a JPEG file is modified and saved the image data is permanently altered. Since image data is saved in 8-bit per color format and lousy compression algorithm is used to reduce the file size, image data can be permanently lost.
In many cases, the photo may only be slightly modified and the loss of image data may not cause noticeable or practically significant degradation in image quality. It also helps to select the highest quality setting when the JPEG file is saved. However sometimes, image quality loss can be more noticeable. For example, if you change the brightness of a JPEG photo and cause the highlight to blowout then save the image, you will not be able to recover the lost highlight details. They are gone!
This is digital photography common sense, right?
Interestingly someone thinks this is a myth. To bust this “myth”, a test was run, where a JPEG photo was opened, nothing useful was done except one pixel was painted white, then the file was saved. The whole loop was repeated 30 times. At the end, no noticeable image quality difference was noticed. And a “myth” was busted.
In practice though, literally opening and saving a JPEG photo without doing anything meaningful is not very useful. For any meaningful photo editing on a JPEG photo, the image data is permanently changed when the JPEG file is overwritten. Changes may not always be good. The changes that you cannot reverse can be very bad.
Keywords: Image Quality, JPEG, Photo Editing, RAW