6 Replies Latest reply: Sep 19, 2011 11:38 AM by Terence Devlin
Chris Shaddock Level 1 Level 1 (0 points)

I have imported a 148KB image 857x1280 at 100ppi into iphoto. When I File> Export at Maximum JPEG quality I get a 475KB image. Same dimensions, same ppi. Can someone explain why please?

 

Thanks

  • Terence Devlin Level 10 Level 10 (134,925 points)

    Easy, because there is less compression used that was used on your original image.

     

    If you want to export your original, use the Kind -> Original option.

  • Chris Shaddock Level 1 Level 1 (0 points)

    My understanding of JPEG is it's a destructive compression (lossy) - meaning pixels are thrown away. Your explainiation makes it sound more like zipping a file. Is that correct? My expectation is if I export as Maximum I would get close to the original but not exactly the original becasue there is still pixels being thrown away. Can you elaborate please?

  • Network 23 Level 6 Level 6 (11,880 points)

    But JPEG compression isn't just about "pixels being thrown away," and it's relative to the original. What could be happening in this case is:

     

    Original was probably saved at a level like JPEG Medium (around 60), since that's pretty common. Data is thrown out, so quite a bit of pixels and color info are downsampled and it drops to 148KB.

     

    Now you open it up and save it out at JPEG Maximum. You are telling it to preserve as much as it can, so it carefully makes note of all the existing data and writes it all out. But the problem is, the existing data was turned into worse data before, so all you did with JPEG Maximum is make it faithfully record a lot of garbage. The reason the file size went up is probably because JPEG cannot use many of the shortcuts it used to save the file out the last time, so even though there's a lot of lower quality data in it, it must still be recorded faithfully (taking up more space) because you set it to Maximum.

     

    This would happen whether you were working with pictures, audio, or video. Rough example: If you take a cheap old webcam video with low resolution and chunky compression, and you add it to a full HD movie, the file size is going to be change to huge HD size, not tiny webcam size. Or if you take a low-bit MP3 and resave it as a high-bit MP3 or as a standard CD file, it is going to get bigger; the sound is no better but the file must grow.

  • LarryHN Level 9 Level 9 (70,680 points)

    You start with a JPEG of one quality and then export it as a JPEG of maximum quality you are decerasing the compression thereby increasing the size

     

    As TD said it you want the original then export the original - you will have an esact copy of what you watarted with

     

    to learn more about JPEGs and how they work see - http://en.wikipedia.org/wiki/JPEG

     

    LN

  • Keith Barkley Level 5 Level 5 (5,765 points)

    Also, the original compression may have added compression artifacts that are *harder* to compress, so that the file might end up being bigger.

  • Terence Devlin Level 10 Level 10 (134,925 points)

    The only time you get a copy of your original file from iPhoto is when you export with the Kind as Original

     

    Any other setting and you re not exporting your file, you are exporting your Photo saved into a new file at whatever settings you choose. So there is no necessary connection between the file import and the file you export. The connection is in the data - that is, your photo

     

    At heart you have a small conceptual leap to make. That Jpeg is not your Photo. It's a file that contains it. Think of it this way. The Beatles wrote a song called Let It Be. They Recorded it. That recording can be stored in an mp3 file. But that mp3 file is not the song. The Beatles didn't write an mp3.

     

    So, your imported file is not exported at a different compression. Your image is exported at a different compression to a different file.