JPG file sizes change just by copying file -- Why?

[Geoff Tani]

jpg files appear to change file size when you copy them to external drives or import them into iPhoto. Could someone please explain why this is?

Thanks to ReunionTalk members, I understand that when jpg files are opened and saved, they are re-compressed; and that re-compression results in some loss of image data. However, I am not opening my jpg files at all. I am copying them without opening them.

In one case, I copy a folder of jpgs from my computer to an external disk (formatted to FAT). Then copy the same file to another disk (formatted to Mac HSF Journaling). The three files have different file sizes. The difference is about 50k. For example, A.jpg is 900k on computer, 825k on FAT, 811k on HFS.

In another case, I import a folder of jpgs into iPhoto on my computer. After doing this, I compare the size of A.jpg in the original folder and A.jpg in iPhoto Library. They are different, in the same way as above -- by about 50k. I found similar (but not the same) drops in file size with iPhoto 6, iPhoto 8, and Aperture.

I do not notice a visible difference in the images. But why does the file size change? And does the difference in file sizes mean that just by copying a jpg file, it loses image data?

Thank you for your help.

[ttl]

Data is stored on hard drives in quantized blocks. Different formatting schemes have different block sizes, so a file with a certain amount of data spread over a number of blocks can end up a different size on different hard drives, particularly noticeable with smaller files.

If you're just copying files, it won't affect the quality of the image. You actually have to open the image in software and re-save the image to force a reapplication of the jpeg encoding.

[Geoff Tani]

Thanks for the information.

May I ask what accounts for the file size changing when I import jpgs into iPhoto on the same computer (same hard drive)?

[ttl]

Sorry, don't have an answer for that. I don't think the image data is being processed, but perhaps some other data is stripped from the file?

[Geoff Tani]

As a possible close to this thread, an Apple Store staff told me the following:

When an image file is downloaded from camera to the Finder, some additional, small and hidden files are added to the image file. Finder adds these small, hidden files for its own purposes. Then when you import those image files from Finder into iPhoto, those small, hidden Finder files are omitted.

So it's not that iPhoto strips out image data during import; rather, it is that Finder adds non-image data during download. Following this logic, A.jpg is 900k in the digital camera; then it is 950k after downloading to Finder; then it is 900k again after importing into iPhoto.