I know that continual resaving of JPG images in an editor such as iPhoto or Elements is bad, resulting in slight deterioration of the image each time. Is there any downside of making Desktop copies of a JPG in the Finder using Option-drag?
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John Russell
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Preventing JPG deterioration
Comments: 3 Responses to “Preventing JPG deterioration”
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JPG images are compressed "lossy" images. That means each pixel saved isn't saved as an individual piece of data. Instead, information is saved about pixels and the pixels around them. So a JPG copy of an image isn't a perfect copy, but it is close and it is much much smaller in file size.
If you open a JPG image, make a change, and export the changed image, it will lose some quality in favor of file size.
So, technically, if you take image 1, open it, export it (not re-save, but export) as a JPG to get image 2. Then do the same thing, opening image 2 and exporting to get 3. Then continuing. You should eventually see a poor image.
But duplicating a file is not the same thing. You are making an exact copy of the file whether it is a text file, Word document, spreadsheet or JPG image. The image is not being reprocessed. So there is no deterioration of the image at all.
Followup question from my wife: When in iPhoto and you choose Duplicate from the Photos menu, does that deteriorate the image or not? Thanks.
No. That makes an exact duplicate of the image. The idea is you can leave one as-is, and then crop or adjust the other one.