Today’s Question: Does Photoshop compress a JPG each time it’s saved, causing continued loss of quality? Do image viewer apps not have this problem?
Tim’s Quick Answer: There is only a cumulative degradation in image quality for a JPEG that is both modified and saved. Simply saving repeatedly without changing the image will not cause further degradation, and simply viewing the image will not cause degradation.
More Detail: JPEG files are almost always saved with “lossy” compression applied, meaning there is some loss of fidelity and quality for an image that is saved as a JPEG image. This degradation from compression obviously applies the first time an image is created, such as when you capture a JPEG image with a camera or save an existing image as a JPEG for the first time.
If you open and re-save a JPEG image without making any changes, the source pixel values have not changed and therefore the JPEG encoding will not be changed, assuming the file settings (such as the Quality setting) have not been changed.
Only when you make changes to a JPEG image and then save it again will there be a cumulative degradation in image quality. Opening an image to view it and then closing without making any changes will not alter the JPEG image. Furthermore, opening a JPEG image and then saving repeatedly without actually making any changes to the image will not cause any degradation. You’d simply be saving the exact same file repeatedly.
The reality is that the cumulative degradation of a JPEG image is not generally a significant issue that you would be able to actually notice even with close examination, provided you used a relatively high setting for Quality. However, because of the general issues of JPEG compression degrading image quality, I don’t recommend using JPEG as a format for saving files that will be modified.
If you are capturing with the raw capture format and need to create a derivative image for editing, I recommend saving as a TIFF or Photoshop PSD file. If you are working with a JPEG capture, I still recommend converting to TIFF or PSD for your working file. I only recommend saving to JPEG when creating a derivative image for sharing with others.