Megabytes, Megapixels, JPEG Compression, and File Size Confusion

We answer this question relating to megapixels, megabytes, and file sizes in the July 2006 Tech Support column of Popular Photography & Imaging:

I want to submit pictures to Pop Photo's Your Best Shot contest, but you require a 3MP or higher (9MB) image size if accepted for publication. I shoot with a Canon EOS Digital Rebel XT and, even with the largest format at 8 megapixels, I can't get a 9-megabyte file size. Does that mean I can't participate? --Arjun Saha

No, it doesn't. Please send your photos. Most digital images have two file sizes. The first is the size of the file that's written to a memory card, usually as a compressed JPEG. This is what camera makers use when they state how many images fit on a card of a given capacity. But when an image is imported into an image editor such as Adobe Photoshop, it assumes a second, uncompressed file size. So, your 8-million-pixel Digital Rebel files will triple to 24 megabytes once they're opened and viewed easily large enough to be published full-page if you triumph in Your Best Shot. Just don't send us RAW files, which take up more space than JPEGs and need post-processing that's your job.

Shortly after the issue went to press, I received a call from a reader wishing to submit photos for Your Best Shot who could not figure out how to make his images look good on-screen, and still meet our 100-kilobyte file size limit.

Guess what? The 100K limit is the compressed file size limit! Just as your in-camera file is smaller than the uncompressed image when opened in Photoshop, the file size you see in the corner of the Photoshop frame is the uncompressed file size.

To clarify this common misunderstanding once and for all (or at least we HOPE so) we're going to use a full-frame image from the 5-megapixel Panasonic Lumix DMC-TZ1 to explain file size and compression variables, from in-camera capture, through saving for screen display.

Check out Image 1. Circled on the File Info box, upper left is the compressed file size of 2.2MB. Check out the uncompressed file size of 14.4MB, circled, lower right, when opened into Adobe Photoshop. If we save this image in an uncompressed format such as TIFF, the uncompressed 14.4MB is the file size that will be saved on your hard drive.

Now take a look at Image 2. This photos has been cropped and resized to 5.25 inches by 7 inches at 72 dpi, which is sized to fit our website. The uncompressed file size (circled) is 558.1K. How do you get this photo under the 100K limit? Easy. Save it as a compressed JPEG!

The JPEG process is a 'lossy' one, meaning that some data is removed, and color gradients are simplified, which may result in some image degradation based on the amount of compression you choose. That is why it makes sense to save your files with JPEG settings that get you as close to the file size limit we specify as possible.