Do you lose quality when you copy a jpeg file ?

[Crouching Tiger]

I'm introducing my Dad to digital photography and somebody at his work has told him that if he makes a copy of a jpeg image, the copy will be of an inferior quality. If he then copies the copy, this will result in a further loss of quality.

I've told him this is utter nonsense. I am right aren't I ? :thinking:

[stephen]

It's a digital file - if you do a straight copy then it will be identical. If you open the file, then save it again, you will lose quality as you will have decompressed it and then compressed it again.

[rehe]

If you load a jpeg into a photo editing program, edit it and save it you will lose qualtiy. This is because they use lossy compression - when you uncompress them you don't get what you started with.

If you copy it in Windows you will not lose any quality.

[DJBenson]

Copy as in a copy/paste jobby - if so, it's an identical digital copy no matter how many times you copy it (for example from camera to hard disk and then to floppy disk and back to CD).

However - editing the file in Paint Shop Pro for example and saving a new copy will degrade the quality as you are compressing a lossy format (JPEG) with another lossy format - same reason you should never reencode WMA to MP3.

Edit : Damn! Too Slow :D

[Dno]

Copying is fine - Ie, Hard disk to Cd, Compact Flash to Hard disk etc...

The guy at work means if you keep resaving a jpeg. For example, if you open a jpeg in photoshop, do some work on it, then save it again you may loose a little quality as the file gets compressed to jpeg format again.

Jpeg is a 'lossy' format, it 'sumerises' the image to keep file sizes down. If your dad wants to edit the photos/is concerned about this, then I'd save the images into a non lossy format, ie, Tiff.

TBH though, as long as the image is large enough/good enough quality in the first place, then your not really going to notice any difference.

[Crouching Tiger]

Thanks guys. Very prompt. I'll pass on the message. :notworthy

[FunkyD]

If it was copying from cd to cd - say thousands of times over wouldn't the image get corrupted?
I am thinking that things like cd recorders can introduce errors in the data they write - which is what the error correction is for.
Some games publishers actually use this to determine whether a game is an original cd or a copy - because when you make a copy the cd recorder will try to correct the errors that were put deliberatly on the cd and so when the game loads it wont see the errors and so know it is a copy...

If you then copy a copy hundreds of times wouldn't the final file become unusable?

[Uncle Nick]

I am thinking that things like cd recorders can introduce errors in the data they write - which is what the error correction is for.

If you then copy a copy hundreds of times wouldn't the final file become unusable?

The answers in the question... error correction. Try googling for the methods of how error correction works ( "Reed-Soloman" IIRC) and you'll see that it's quite a complicated and robust process. Add onto that the fact that Windows *will* spit it's dummy out if it gets EC errors from a data transfer, and your data is reasonably well protected.

[Kryten]

Moving...