Q&A: Mysterious JPG File Sizes

I frequently use a program like Adobe PhotoShop to edit my digital photos. Recently, though, I noticed that the file size changes dramatically when I save it. I might start with a JPG file that's around 800KB, for instance, but after saving it the file will be 500KB or less. Am I doing something wrong?

-- Rich Goreman, Mountain View, California

There must be something in the air right now, because I received several e-mails this week asking essentially this same question.

There could be several things afoot. First of all, if you make significant changes to an image (like resolution, cropping, or brightness/contrast) the JPG file size can change quite a bit. That's nothing to worry about; it's just a reflection of the fact that your image is no longer the same as it was.

Another factor is the JPG compression level. The original JPG image--saved by the camera--and the new JPG, saved by your image editor, might have been compressed at two very different quality levels. Your digital camera may only have three JPG quality levels, such as low, medium, and high. Adobe Photoshop, however, has a dozen levels. Paint Shop Pro has a whopping 99 levels of compression.

As always, I recommend that you save your edited images in TIFF format or the highest quality JPG compression level you can choose. Your eyes are the real test, though. If the image looks goods to you, I wouldn't be too worried about variations in file size when you save an image after editing.

Send your questions to question@bydavejohnson.com, and please be sure to let me know where you're from.

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