Questions about raw, tiff, jpeg, and DNG

Started Mar 27, 2013 | Questions thread
Hugowolf
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Re: Questions about raw, tiff, jpeg, and DNG
In reply to Tom Axford, Mar 27, 2013

Tom Axford wrote:

Hugowolf wrote:

Tom Axford wrote:

JPEG images incur a loss of information every time a JPEG is saved.

This is a myth, and can easily be shown to be incorrect. If you an edit between saves, even a minor one pixel edit, then you can incur data losses at each save, but simply saving the file using the same compression and resolution parameters should not cause further data loss.

It is an easy, but tedious, test to do. And since I have seen the results before, I am not going to repeat it.

Take an image and open it in Photoshop. Save it as a Jpeg with whatever parameters you like. Make a duplicate of the image and save it with the same parameters, then hit ctrl+shift+S (save as, then enter, enter, enter) a million times. Copy the millionaire duplicate onto the original as a layer and change the blending mode to difference. There should be no difference.

Brian A

There is no point whatsoever in loading and saving the same image many times in succession. Many programs are intelligent enough to realise this and keep the original image all the time, so they don't bother to do the reload (as the image has been kept in an internal buffer).

If, instead, you save the image, then quit the program, then restart it again and reload the saved JPEG, do a bit more editing and save again, and so on,... after a few cycles of this process you will begin to see a degradation in quality (more noticeable if you set a fairly low JPEG quality).

I learnt this myself the hard way - the loss of quality became very obvious after about 10 cycles of editing and saving. Always save as a TIFF if you want to continue editing later.

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

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