Questions about raw, tiff, jpeg, and DNG

Started Mar 27, 2013 | Questions thread
Tom Axford Veteran Member • Posts: 4,275
Re: Questions about raw, tiff, jpeg, and DNG

Hugowolf wrote:

It has nothing to do with quitting and restarting the program, it is only the editing that matters. Photoshop won't let you save an unchanged image in the same format, the 'save as' actually saves over the old image file. You could write a script that openned, resaved, and closed several times. It will not have an effect on the image quality.

(And, there are lossy forms of TIFF).

Brian A

I've done some quick tests and I have to admit that you are correct if the image is not edited (or edited in ways that do not affect most of the image).  I think the JPEG algorithms have been improved over the years, because that wasn't the case many years ago when I first encountered this problem.

However, in some cases, the image quality still degrades each time you save a JPEG.

To illustrate this, I created an image of the text "AB" on a white background and saved it (at the lowest JPEG quality). I then reloaded the image, edited it (by slightly cropping the image size) and saved the result. This was repeated once more. The three saved images are shown below. You can see how the quality degrades further each time. I think this is because the slight cropping effectively changes the whole image so the JPEG algorithm compresses the image quite differently.

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