Compression Clarification


Today’s Question: Am I correct that image degradation to a JPEG only occurs if some sort of file manipulation takes place and it is re-saved? But if a file is just opened to see the contents and then re-saved (closed) without any change then the image is not subject to further lossy changes? And what about JPEG 2000?

Tim’s Quick Answer: Yes, you are correct about JPEG compression. A JPEG image always represents some degree of quality loss compared to a non-compressed image. However, additional degradation of image quality will only occur if the image is changed and then saved. Re-saving the same image with no changes will not result in additional quality loss. And JPEG 2000, by the way, provides improvements over JPEG (including an option for lossless compression), but has not been widely adopted.

More Detail: JPEG compression is always “lossy”, meaning some degree of quality will be lost when a photo is saved in the JPEG format. The Quality setting for the JPEG image determines the degree of compression applied to the image, and thus the degree to which quality is lost.

If you make changes to an image and then save it again as a JPEG image, there will be an additional loss of quality. However, that loss in quality only applies if the pixel values were changed. So, if you open a JPEG image, don’t make any changes, but re-save the same image multiple times, there will be no additional loss of quality for the image.

But again, making changes to the image and then saving again as a JPEG will cause an increased degradation in quality for the image compared to the “original” version of the JPEG, because the compression would then be applied again to the image.

The JPEG 2000 file format provides advantages in terms of compression and image quality compared to the JPEG file format. However, JPEG 2000 has not been widely adopted and therefore is not supported in all software applications. It is supported by Photoshop, for example, but not Lightroom.