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Old Jan 16, 2009, 1:02 PM   #1
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Ive searched this and got conflicting information. I purchased a book after I got my first dSLR over christmas and found out jpegs lose quality as you "resave" them. The author suggetsed each time you copy and save them they lose image quality. I think that's the exact wording, indicating it doesn't require a resave but simply a cut/paste to do this. I was confused by this because I know computers, not cameras, but thats really just a reindexing assuming you're not switching drives and the file isn't actually be physically moved most of the time.

BUT, it would be moved in other circumstances when you dont actually move it yourself, such as when optimizing a drive. I see no reason windows would open and recompress the .jpg at this point but sometimes windows does crazy things, I thought it just bit for bit copied it and any image or file should retain 100 percent likeness assuming an intact drive. Otherwise we'd have constant issues with computers breaking due to lost data in various programs.

Anyways, so i searched around on the internet and got conflicting answers. Some say cut/paste or copying the file doesnt degrade it, others say it does. Does anyone have a definitive answer? I want to shoot in RAW but the fact is on long trips I dont have enough cards to do that and I dont go places I can bring my laptop and leave it safely. I also dont intend to do post work on most photos, when I go on vacation I take lots of photos, and lots of them arent supposed to be great, just quick snaps, so jpg is fine. When Im really trying to learn and advance my hobby of photography I take the time and switch to raw so i can have some fun after with it. Im really just concerned that over the course of 10 - 15 years with me switching computers (I get a new one every 1 to 1.5 years) that my pictures will turn to trash.

So, after my windy question/explanation, is this a gap in some photographer's knowledge of computer language and jpgs only degrade when creating a new file (open it, click save as), or do they truly lose quality with every save made (cut/paste and copies are saves)?
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Old Jan 16, 2009, 1:55 PM   #2
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Copying or moving JPEGs will not degrade the image. The copy/moved file will be an exact copy of the original - unless there is corruption which is *MUCH* more likely to make the file unreadable than degraded.

Resaving a JPEG will in theory degrade the image, but if the compression is not severe, it is not likely to be noticable until it is done several tens or hundreds of times.

There is a simple test - just put the copy on top of the original and select the blending option to difference. In the first case, all pixels will have a value of zero. In the second most will have a value of zero and the others will be very small. Just look at the histogram to verify.
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Old Jan 16, 2009, 2:12 PM   #3
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All JPEG files are compressed to some degree, and, unlike .ZIP files, the compression is not lossless. Each time you open a JPEG in an image editting program, the image is decompressed, and if you save it again, it is compressed again, and some of the original detail is irretreivably lost. So, each time you save an image to a JPEG file, you lose a little more detail. The amount of detail lost depends on the compression setting used, and if you set the compression to minimum compression/maximum quality, it might actually take a hundred or so saves before you actually notice the degradation. That's what the author is talking about.

What you're talking about is copying and pasting a file. That does not degrade image quality. The copy containsexactly what the original file contained, because the image has not been decompressed and recompressed.
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Old Jan 17, 2009, 5:44 PM   #4
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Thanks for the answers
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