Thursday, 28 May 2009

More on jpeg settings

Walcott beach, Norfolk, May 2009
From Lightroom, jpeg quality 50, 1200x800
click for full size

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.

Lightroom jpeg quality 10, 1200x800

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.

Gorse, Cumbria, May 2009
Lightroom jpeg quality 50, 1200x800

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.


  1. I think you are also missing the potential for zooming an image on a screen. Maybe 1200x800 is fine if it'll only ever be viewed at 100% (and in about half the screen), but just like you might get closer to a big print, or view it from a distance, zooming in or out on a screen is trivially easy. Better to publish at a slightly higher resolution than you expect it to be viewed at.

    As it is, a 1200x800 resolution book would cover about 2/3rd of my current screen - if published at that resolution it is going to look quite soft viewed full screen and not really support any detail viewing at all.

    It is also probably a mistake to assume that most photographers use screens the same size as most internet users. I'd suspect that there would be a tendency for photographers to be towards the top of any resolution bell curve, rather than in the average resolutions.

    1200x800 seems pretty low, if you take zooming and common resolutions into account.

  2. Gordon, certainly there are exceptions to every rule and if you know what you're doing then fine (and I well know you're on top of this stuff). Sure, photographers are more likely to have larger screens. Personally, I don't think zoomable images add much.
    But I was really aiming this squarely at the SoFoBoMo crowd. When I look at the published books, the files are huge but the pictures aren't.
    And I think my last point is one of the most important - even if photographers have large screens, they don't necessarily have super-fast internet connections. If the same content can be made a third to a half the size, everyone wins, IMO.

  3. yup - I'm not really disagreeing. I suspect many people are actually just using full resolution images, maybe with very minimal compression.


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