Bison Bud wrote:
Maybe a stupid question and I think that I know the answer, but I'm going to ask it here for clarification anyway. We all know that the .jpeg format compresses the picture data and that the data deleted is gone forever unless otherwise backed up with another type file that doesn't use compression. This can be a good thing or a bad thing depending on how you look at it. Anyway, this compression obviously takes place when the original edited RAW file is converted to .jpeg, but does this also happen every time I make a copy of the .jpeg file? If I do a simple file copy to move a .jpeg file from one location to another, do I get the complete original .jpeg data or is it compressed again even if the picture is unedited?
Maybe a stupid question and I think that I know th... (
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You have to RE-SAVE a JPEG to RE-COMPRESS it.
If you just copy the file, using the operating system, you get an exact digital copy.
If you open and print the file without saving it, nothing changes.
If you open and view the file and then close it without saving it, nothing changes.
But if you open a file and then SAVE it or EXPORT it as a JPEG, you lose data.
That said, the operant question is always, "Does it matter?" It can matter a lot, or a negligibly little amount, depending on the source of the original file (adjusted in a raw converter vs. straight-out-of-the-camera), the scene itself, and how good the JPEG was to start with. Did you save the highest quality, largest file size possible? Were your raw adjustments made perfectly, in reference to a calibrated and ICC profiled monitor? Did you set the camera menus (ALL of them!) perfectly for the scene?
The irony of JPEG photography is that the closer you are to perfect exposure and white balance at the camera, the more latitude you have to adjust a straight-out-of-the-camera JPEG later.
The largest data loss is on conversion from raw data, regardless of where that happens. So if you have accurate exposure and white balance, and adjust the menu settings correctly for the scene, little visible data will be lost... IF the scene brightness range can be captured using the menu settings and exposure settings available.
That said, there is one helluva lot more data in a raw file. It may be impossible to save it all at the camera, due to the limits of the camera's JPEG processing menus. But manipulating the raw image in Lightroom or a similar software may give you a much better JPEG image, because of the range of control and the wider range of available tones.