JPEG compression

Discussion in 'Digital Photography' started by James Ramaley, Oct 25, 2004.

  1. My camera has various JPEG compression modes. The manual says that
    higher compression means lower image quality, but I haven't seen any
    difference in the image quality of the two compression levels that I
    have been using: FINE (1:4) and NORMAL (1:8). However, I did see a
    signigicant difference in file size. What am I missing here?

    thanks
    James Ramaley, Oct 25, 2004
    #1
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  2. James Ramaley

    Jim Townsend Guest

    James Ramaley wrote:

    > My camera has various JPEG compression modes. The manual says that
    > higher compression means lower image quality, but I haven't seen any
    > difference in the image quality of the two compression levels that I
    > have been using: FINE (1:4) and NORMAL (1:8). However, I did see a
    > signigicant difference in file size. What am I missing here?


    You're not missing anything :)

    JPEG is an extremely effective method of compression. It's done
    by discarding some of the color information.

    If there are say, 20 pixels adjacent to one another that are very
    close to the same color, it *will* make them the exact same color.
    Once that is done, it can represent these 10 pixels as a few bytes
    rather than 30 bytes. (This is a *very* rough description of how it
    works :)

    If you look at the size your images, you'll see the ones with large areas
    of the same color (ie a large smooth wall) compress better than images with
    lots of detail. (A gravel road).

    The reason this works is that in a photographic image, these changes are
    too subtle to be seen by the eye... Of course this is only true to a certain
    level.. If you compress too far, the images will be noticeably worse.

    There *will* be a difference in images with different levels of
    compression, but you'll have to blow them up to 400% or more and look
    hard to really see the difference.

    Look at the areas of the image where there are large transitions
    between light and dark. You should see some dark distortion there.
    This is called 'artifacting' Another thing to look for is banding
    in large areas of similar color. (like the sky).
    Jim Townsend, Oct 25, 2004
    #2
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  3. James Ramaley

    SleeperMan Guest

    James Ramaley wrote:
    > My camera has various JPEG compression modes. The manual says that
    > higher compression means lower image quality, but I haven't seen any
    > difference in the image quality of the two compression levels that I
    > have been using: FINE (1:4) and NORMAL (1:8). However, I did see a
    > signigicant difference in file size. What am I missing here?
    >
    > thanks


    Best option to see the difference is to shoot some text and see it - but
    usually you'll have to look it magnified, to see the difference.
    There's distortion around text. But i agree that there's barely noticeable
    difference between fine and normal, there's a bit more between normal and
    coarse(or low, whatever your camera says).
    Interesting point is that cameras use exactly opposite of usual words -
    usually it's said High as high compression (and so low quality), while all
    cameras have high as high quality...
    SleeperMan, Oct 25, 2004
    #3
  4. James Ramaley

    Gadgets Guest

    More noticeable on straight edges and fine detail - signs, textures etc.
    Zoom in enough and you'll see the difference, but at 'normal' scales it's
    probably so minor a difference as to be negligible. Maybe becomes more
    noticeable if you have to lift the shadows too...

    Cheers, Jason (remove ... to reply)
    Video & Gaming: http://gadgetaus.com
    Gadgets, Oct 25, 2004
    #4
  5. James Ramaley

    Ron Hunter Guest

    James Ramaley wrote:
    > My camera has various JPEG compression modes. The manual says that
    > higher compression means lower image quality, but I haven't seen any
    > difference in the image quality of the two compression levels that I
    > have been using: FINE (1:4) and NORMAL (1:8). However, I did see a
    > signigicant difference in file size. What am I missing here?
    >
    > thanks


    There are differences, but you might not be aware of them in any single
    photo. Try compressing a picture of your lawn at both compressions
    levels, and then compare them blown up onscreen to 200%. That should
    show you how they are different, and what effect excess JPEG compression
    has on the image quality.
    Ron Hunter, Oct 25, 2004
    #5
  6. James Ramaley

    GT40 Guest

    On 24 Oct 2004 22:30:16 -0700, (James Ramaley)
    wrote:

    >My camera has various JPEG compression modes. The manual says that
    >higher compression means lower image quality, but I haven't seen any
    >difference in the image quality of the two compression levels that I
    >have been using: FINE (1:4) and NORMAL (1:8). However, I did see a
    >signigicant difference in file size. What am I missing here?


    It all depends on what size your viewing at. Zoom in on the image,
    and you'll soon see.
    GT40, Oct 25, 2004
    #6
  7. (James Ramaley) writes:
    >My camera has various JPEG compression modes. The manual says that
    >higher compression means lower image quality, but I haven't seen any
    >difference in the image quality of the two compression levels that I
    >have been using: FINE (1:4) and NORMAL (1:8). However, I did see a
    >signigicant difference in file size. What am I missing here?


    Try shooting a high-contrast subject, like bare tree branches against an
    overcast sky. Then look at the images at 100%. You'll probably see
    some artifacts along the boundary between tree branch and sky in all
    of the images, even the highest-quality one. In most photos, the effect
    is less visible because the contrast is less, but now that you know what
    to look for you'll see it in other places.

    I did this once with my G2, and decided that memory cards and CDRs (for
    archiving) are cheap, so I always shoot in either RAW (no lossy
    compression at all), or JPEG at maximum size and least compression
    (Superfine). I've never used the higher-compression JPEG modes except
    for these tests.

    Dave
    Dave Martindale, Oct 25, 2004
    #7
  8. James Ramaley

    DHB Guest

    On Mon, 25 Oct 2004 00:50:52 -0500, Jim Townsend <>
    wrote:

    >James Ramaley wrote:
    >
    >> My camera has various JPEG compression modes. The manual says that
    >> higher compression means lower image quality, but I haven't seen any
    >> difference in the image quality of the two compression levels that I
    >> have been using: FINE (1:4) and NORMAL (1:8). However, I did see a
    >> signigicant difference in file size. What am I missing here?

    >
    >You're not missing anything :)
    >
    >JPEG is an extremely effective method of compression. It's done
    >by discarding some of the color information.
    >
    >If there are say, 20 pixels adjacent to one another that are very
    >close to the same color, it *will* make them the exact same color.
    >Once that is done, it can represent these 10 pixels as a few bytes
    >rather than 30 bytes. (This is a *very* rough description of how it
    >works :)
    >
    >If you look at the size your images, you'll see the ones with large areas
    >of the same color (ie a large smooth wall) compress better than images with
    >lots of detail. (A gravel road).
    >
    >The reason this works is that in a photographic image, these changes are
    >too subtle to be seen by the eye... Of course this is only true to a certain
    >level.. If you compress too far, the images will be noticeably worse.
    >
    >There *will* be a difference in images with different levels of
    >compression, but you'll have to blow them up to 400% or more and look
    >hard to really see the difference.
    >
    >Look at the areas of the image where there are large transitions
    >between light and dark. You should see some dark distortion there.
    >This is called 'artifacting' Another thing to look for is banding
    >in large areas of similar color. (like the sky).
    >


    Jim Townsend,
    great job @ describing JPEG photo compression in an
    easy to understand way. You did such a good job that there is little
    else to add but I will offer a related side bar.

    If 1 believes or knows that a given picture(s) might or will
    need to be heavily cropped or a very large print made of them, then
    selecting the lowest JPEG compression (best picture quality) is the
    way to go. However if you should find yourself running low on memory
    card storage space @ a given photo shoot & are using the lowest
    compression, switching to a higher compression to fit more photos in
    the remaining space on the memory card becomes a viable option.

    Often people ask if switching to a smaller size (resolution)
    would be better. Since JPEG works so well in most cases, it's better
    to remain @ the higher resolution & increase the JPEG compression to
    reduce the file size. This is largely true because many of the
    artifacts can later be edited out but lost resolution can not be
    replaced. However we are fortunate that advances in flash memory is
    continuing to drive memory priced down, speed & capacity up. So to
    many it may seem like a 512MB or a 1GB memory card is very large, but
    in a year or so, they are likely to be considered the smallest size
    worth buying or using in many situations.

    Last note, consider also the price/size ratio of any memory
    card. With higher resolution cameras becoming increasingly common,
    larger cards are also becoming more common & less costly. Thus it may
    be lass expensive to purchase a 1GB card rather than 2 512MB cards.
    Yes somebody will point out the logic of "not having all you eggs in 1
    basket" & there is some validity to this even with very reliable
    memory cards but the same applies to a hard drive in a computer. Most
    consumer computers have only a single hard drive in them, "all there
    eggs in 1 basket". Yes you can & should back-up this DATA
    periodically but unless your running 2 drives on a RAID system, if
    your drive suddenly suffers an unrecoverable failure between back-ups,
    you have lost all of that information added since your last back-up.

    There is no 100% safe way to store your DATA of any kind but
    spreading it out & backing up often can greatly minimize the chance of
    loss. This is also why "I" don't buy generic memory from the cheapest
    vendor. Yes most of that is highly reliable as well but I prefer to
    only buy from a manufacture that offers a "lifetime warrantee". Lexar
    12X & faster CF memory cards all have such a "lifetime warrantee" as
    do other companies.

    Best of luck & again as a general rule, my preference & advice
    is to almost always shoot @ the highest resolution & minimum JPEG
    compression whenever possible. However as noted above, I will
    increase compression if I am running low on memory storage space or if
    I an almost certain that the picture will never be printed or need to
    be printed any larger than 5x7 cropped more than a little".

    Respectfully, DHB
    DHB, Oct 25, 2004
    #8
  9. (James Ramaley) writes:

    > My camera has various JPEG compression modes. The manual says that
    > higher compression means lower image quality, but I haven't seen any
    > difference in the image quality of the two compression levels that I
    > have been using: FINE (1:4) and NORMAL (1:8). However, I did see a
    > signigicant difference in file size. What am I missing here?


    There will be large differences in quality between those modes. You
    probably won't see them until you view the image at 1:1 magnification
    (actual pixels) on screen. You'll spot it most easily in large areas
    of fairly even color, like a blue sky.
    --
    David Dyer-Bennet, <mailto:>, <http://www.dd-b.net/dd-b/>
    RKBA: <http://noguns-nomoney.com/> <http://www.dd-b.net/carry/>
    Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/>
    Dragaera/Steven Brust: <http://dragaera.info/>
    David Dyer-Bennet, Oct 25, 2004
    #9
  10. James Ramaley

    bob Guest

    (Dave Martindale) wrote in news:clj5tj$9ho$1
    @mughi.cs.ubc.ca:

    > Try shooting a high-contrast subject, like bare tree branches against an
    > overcast sky. Then look at the images at 100%. You'll probably see
    > some artifacts along the boundary between tree branch and sky in all
    > of the images, even the highest-quality one. In most photos, the effect
    > is less visible because the contrast is less, but now that you know what
    > to look for you'll see it in other places.
    >


    I think tree branches are about the best place to spot artifacts, but
    telephone wires work too.

    In the tree branches, you will frequently see dark blue mush in the higher
    compressions where there are black details in the low.

    Bob

    --
    Delete the inverse SPAM to reply
    bob, Oct 26, 2004
    #10
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