Thursday, 28 May 2009
Ed Richards had comments on my suggestions on jpeg over at Paul Butzi's. So, as is my wont, I went off to test his points (and mine for that matter).
A few points to start: if one is creating jpeg images for displaying on a screen, then a lot of the original data is thrown away anyway. For a typical 10MP original, going to 1200x800 (I'll work with Ed's big image assumption) throws away about 90% of the original pixels. If you're an LF photog, it's going to be way more than that. So we can forget about on-screen versions ever getting to the fine detail and nuance of a really nice print from a really nice image.
The next idea is that higher quality settings really have an impact on the on-screen viewing quality. If I want to show the very best, I need the higher quality. But how true is that?
I ran a bunch of tests on a series of images with different jpeg quality settings. For this I used Lightroom but any jpeg generator would yield similar results. I ran film & digital originals. I tried detailed & mixed images (with some expanses of limited detail). I tried jpeg quality settings from 10 to 95. I visually compared the results versus quality setting and file size.
What did I find? Obviously, at low settings there were problems. Lots of artefacts around edges, detail blurring, pixelation in large, continuous areas. At quality of 30-40 most problems were gone. Some fine details started to go. At 50 most images were indistinguishable from the higher settings. The few differences needed a careful look and hopping between versions. Going from 70-95 showed no improvement on any image.
How about file sizes? At 1200x800, quality of 50 gave about 220kB per image. At 70 that was 350-400kB and at 90 all the way up to 1.5MB. Yet no visible difference between them (and I was looking carefully on a decent, calibrated monitor). 1000x667 images were proportionally (by area) smaller.
Of the hundreds of images I've posted, the average file size is around 250kB and I can't recall seeing compression artefacts or loss of detail in any of them at web sizes.
Like I say, for good, on-screen display, target a file size of around 150kB for 1000x800, up to 200kB-ish for 1200x800. The reason I've been suggesting 1000x800 is that's a 10"x8" at 100ppi, which is a decent size for a book image, working on the basis that we're talking embedded jpegs in a pdf.
Ed also made a point that he wants the highest quality to wow the guys with the huge, high quality monitors. Thus he ends up with a big pdf. Personally, I want a bunch of people to download my SoFoBoMo book, so I'd rather a smaller file size to encourage them to do so. And based on the above tests, I reckon a 5MB book file would be visually indistinguishable from a 15MB one anyway, on any screen.