Q&A: More Questions About JPEG Compression

Like many people, I have a lot of old slides and negatives that I need to scan into my computer, and I usually do this in such a way that a 35mm slide ends up as a digital image with about 3000 pixels on the long side. I'm saving these scanned images as JPEG files because they take up much less space on my computer than TIFF files, but I'm not sure if that's the best format for recovering and storing the maximum available detail from the original. Do I lose pixels to compression when I scan to a JPEG, or do I only lose pixels when I resave a JPEG?

--Graham Jacks, Hamilton, Ontario

That's a good question, Graham. Saving your scanned photos as JPEGs really will save a huge amount of space: A single TIFF at the resolutions you're scanning can require several dozen megabytes, while an equivalent JPEG might be just a couple.

But your question misses the mark just a bit. JPEG compression never makes you lose pixels. No matter how many times you resave a JPEG or how much compression you use, you'll always have exactly the same number of pixels in your picture. Instead, the problem involves color accuracy: File compression introduces color glitches and other visual errors into the picture.

For most people, though, all this is just academic: If you save your JPEGs at the very highest quality level, you can avoid visible compression defects in your images and get TIFF-like results. That said, the first time you create a JPEG image, it loses some color accuracy to the compression process. Then, every time you resave a JPEG, the file is recompressed, further reducing the image quality. That's why, if I were in your shoes, I'd always set my software to save (and resave) your JPEGs at the very highest image quality.