change ISO vs increase RAW exposure

Discussion in 'Digital Photography' started by G Duran, Mar 19, 2005.

  1. G Duran

    G Duran Guest

    in terms of digital noise:
    is it the same to increase the iso in the camera before taking the
    picture that take the photo in RAW with the lowest iso and then
    increase the exposure with the RAW-reader program?
     
    G Duran, Mar 19, 2005
    #1
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  2. G Duran

    JPS Guest

    In message <>,
    Unless the camera's analog-to-digital converter is really bad, it is
    always better to saturate the range of RAW values at a higher ISO than
    to under-expose at a lower ISO. Under-exposure should be avoided
    whenever possible, as it causes posterization of the signal and the
    noise. I only under-expose when there just isn't enough light, such as
    shooting at the highest ISO at night.

    A well-exposed image at ISO 1600 looks orders of magnitude better than
    the same shot at ISO 100, relatively under-exposed by four stops (same
    aperture and shutter speed).

    This is very important to understand if you want maximum quality
    captures. When the contrast of the scene is low, crank that ISO, and
    boost the exposure compensation, instead of exposing in the middle zone
    at ISO 100. Just watch out that you don't clip important detail in the
    RAW capture.
    --
     
    JPS, Mar 19, 2005
    #2
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  3. G Duran

    Stacey Guest

    Normally no but:

    Depends on the camera and the max designed ISO of the sensor. If the camera
    is "boosting" the sensor to get higher ISO setting, then a lower ISO
    underexposed a stop then increasing the exposure in RAW can get a cleaner
    image. An example of what I mean:

    On my olympus E300 (shooting RAW): a native ISO 800 shot is cleaner than a
    ISO 400 one underexposed 1 stop and then increasing the exposure a stop in
    developing to end up at the same place. On the other hand it does produce
    cleaner results shooting at ISO 800 1 under and pushing the exposure a stop
    in development than shooting at native ISO 1600.

    Only testing for yourself will show you if this works for your camera/sensor
    and is probably only worth doing at the highest ISO's.
     
    Stacey, Mar 19, 2005
    #3
  4. G Duran

    secheese Guest

    Interesting! Are you suggesting that for a normally lit scene (ie. no
    difficult exposure issues) and all else being equal, that it's best to
    shoot at 1600 vs. 100; unlike the film world?

    Can you elaborate or site a source I could read?
     
    secheese, Mar 19, 2005
    #4
  5. You could start by reading the message you quoted more carefully...

    David J. Littleboy
    Tokyo, Japan
     
    David J. Littleboy, Mar 19, 2005
    #5
  6. G Duran

    JPS Guest

    In message <>,
    What I am suggesting is that for any given combination of aperture and
    shutter speed, you will get a cleaner image with the highest ISO that
    doesn't blow out the RAW data. For example, if you find yourself
    shooting in a dense fog, and the aperture and shutter speed are at
    usable limits at ISO 400, you will get a better capture at ISO 1600 and
    +2 EC, than you will at ISO 400 with 0 EC, using the exact same shutter
    speed and aperture. The exposure on the sensor is *exactly* the same
    for each; except for determining automatic exposure and manual metering,
    the ISO has no affect on exposure of the sensor. Its only affect is the
    scaling of the analog-to-digital conversion.

    I've tested this in many scenarios on both the 20D and 10D, and with the
    same absolute exposure (aperture and shutter speed) the higher ISO image
    is always as good or better than the low-ISO shot; never worse.

    Some people understand intuitively that grossly under-exposing at ISO
    100 is to be avoided, but few people realize that a low-contrast scene
    with moderate light can also benefit from going to a higher ISO, with +
    EC. Film thinking still rules the day. Both film and digital get
    grainy/noisy from under-exposure. Only film gets low-contrast (and
    therefore, grainy upon restoration) from over-exposure. Digital records
    better right up to the point before it clips.

    Of course, if you allow the exposure to vary automatically in the
    different ISOs, then the lowest ISOs will be cleanest (but may sacrifice
    DOF or action-stopping to shoot at that ISO).
    --
     
    JPS, Mar 19, 2005
    #6
  7. G Duran

    Zed C. Pobre Guest

    Short answer: you would never want to shift from ISO100 to ISO1600
    on a shot that you can expose correctly at a decent shutter speed at
    your desired aperture, but you might often want to shift from 100 to
    200, or from 200 to 400 to deliberately overexpose slightly (or avoid
    mild underexposure), and then correct that in post-production.

    For the longer answer, you have to understand the two main issues
    involved.

    First, the amount of bit depth allocated to storing the values in
    an image is not linearly spread across the entire brightness range.
    There is three times as much information available at the upper two
    stops than there is in the lower three, given a camera with a 5-stop
    dynamic range. You might want to read "Expose to the Right" and
    "Understanding Histograms" by Michael Reichmann
    (http://www.luminous-landscape.com/tutorials/expose-right.shtml and
    http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml).

    Second, there is the issue of how much noise is introduced at each
    ISO level, and how much blur is introduced by a slower shutter speed
    in a handheld shot or with a less than perfectly stable tripod (for
    very long exposures).

    If you know in advance that you are going to post-process from the
    raw image, you want to avoid underexposure when possible, and even
    slightly overexpose in some cases, as long as you don't blow your
    highlights. When possible, get extra light by going to a slower
    shutter speed. If you are shooting by hand, and have run up against
    your minimum speed for a sharp image, then you get to consider the
    quality of your sensor.

    A Canon EOS 20D has excellent progression up to ISO1600, but then
    a sharp degradation at "H" or ISO3200. When I shoot, I'll bump the
    ISO up as far as 1600 to avoid even slight underexposure or need to
    take a faster shot (for instance because I need extra depth of field
    by decreasing aperture or because the subject is moving), and will go
    from 100 to about 400 if I want to put more data in the upper light
    zones even if the shot is exposed decently. I will go to 3200 if I'm
    losing the shadows (I take a shot, and the histogram shows that I'm
    pressed up against the left hand side), but I try to avoid it except
    in desperate cases. Adding noise is almost always superior on a
    decent camera to completely losing information.

    There's something of an extreme example of this for when I did a
    review of night shots with the 20D a little while ago at
    http://www.resonant.org/node/471 (check for the image that is dead
    black at ISO400, and compare how much I was able to retrieve with
    level adjustment, then compare again to the ISO3200 version, and the
    level adjusted version of that).

    Whether or not going to a higher ISO to get the extra information
    when a shot is only slightly underexposed or exposed correctly but
    with a lot of dynamic range to spare on the bright side requires a bit
    of personal judgement. My recommendation, given that storage is
    becoming cheap, is to do both whenever you're in doubt. Take a shot
    at the lowest ISO where you are not significantly underexposed for the
    shutter speed and aperture that you want. Put both shots through
    Noise Ninja, adjust levels, curves, and/or shadows/highlights, and
    then compare the results. After a while of doing this, you should get
    a feel for when it's a good idea.

    That's a decent general rule of thumb, actually: when in doubt,
    take lots of pictures. :)
     
    Zed C. Pobre, Mar 19, 2005
    #7
  8. G Duran

    JPS Guest

    In message <>,
    There is no ISO 3200 mode, per se, on the 20D. It is using ISO 1600,
    but exposing for 3200, and doubling the digitized RAW numbers ... they
    are all even except for half of the bad pixels, which are interpolated
    to odd numbers. They are aboout 99.9% even.

    ISO 3200 on the 20D is really a JPEG mode, as shooting RAW at 1600 and
    -1 EC captures everything RAW 3200 does, plus it gives you one more stop
    of highlights. For JPEGs, the story is different, as the extra
    highlights are clipped away, anyway, and it is better to have the image
    normally-levelled before conversion to 8-bit JPEG.

    This is probably true of most if not all current digitals that start at
    ISO 100 and go to 3200. The 10D's "1600" is 800, and its "3200" is
    1600, done the same way. The difference is that on the 10D Canon
    appears to be trying to fool histograms and RAW peepers by striping the
    RAW data with solid and dotted horizontal lines that are all offset to
    odd numbers, but the bottom line is that any position in the RAW bitmap
    can only be one of 2048 odd or even numbers.
    --
     
    JPS, Mar 20, 2005
    #8
  9. G Duran

    Bill Hilton Guest

    G Duran writes ...
    I've tested this on a couple of cameras and I have to disagree with
    those who are seeing "orders of magnitude" improvement. I'm seeing
    roughly equivalent noise when I do this. Keep in mind that if you
    increase ISO and then add exposure compensation you'll have a lot more
    editing in the RAW converter than if you simply shoot at the correct
    exposure at lower ISO. The other thing to worry about is clipping
    highlights, which is much easier to do when you're working close to the
    right edge of the histogram.

    This is based on what I see with my cameras, you might get different
    results with a different camera or a different RAW converter. So I
    suggest you run some tests with your gear and decide for yourself if
    it's worth the hassle. Here are results from such a test from one of
    my cameras ...

    First, I shoot a neutral subject (narrow tonal range) at each ISO at
    the metered reading, then with -2, -3, + 2, and +3 stops compensation
    (the cameras I use only go +/- 3 stops unless you expose manually).
    Here's a screen grab of what these RAW thumbs look like in the RAW
    converter before any RAW processing is done ...
    http://members.aol.com/bhilton665/thumbs.jpg

    This next link shows noise samples from four files, ISO 800 metered and
    unchanged during RAW conversion, 800 underexposed by two stops and then
    adjusted during RAW conversion for an equivalent exposure, 800
    overexposed by 2 and adjusted down by 2 during RAW conversion for an
    equivalent exposure, and ISO 200 as metered. The three ISO 800 shots
    show the benefits of "exposing right" since the +2 exposure with -2 RAW
    conversion has much less noise. But if you compare this +2/-2 @ 800
    crop to the iso 200 crop you won't see much difference in noise. These
    are all processed with the default setting in my RAW converter, 100
    pixel crops from the center of the file and then blown up to 400% to
    show any noise. At 100% (web view) any difference between 200 and 800
    @ +2/-2 is neglible. I see basically the same trend comparing iso 100
    metered to 400 +2/-2 and 400 metered to 1600 +2/-2.
    http://members.aol.com/bhilton665/iso800_200.jpg

    One more link ... this one shows iso 800 +3 exposure with -3 RAW
    (actually -2.2 ec to get there). Again, there is little discernible
    difference in the noise. I also included the histograms and as you can
    see there is little room for error with the +3 shot since the histogram
    is heavily biased to the right. This is shooting a gray card, most
    "real life" scenes would have more contrast and would likely clip the
    highs with +3 compensation.
    http://members.aol.com/bhilton665/iso800_100.jpg

    These tests used a professional camera with pretty low noise levels
    (Canon 1D Mark II) and a very good RAW converter (Capture One Pro).
    With smaller, more densely packed sensors and different converters you
    might get different results.

    Bill
     
    Bill Hilton, Mar 20, 2005
    #9
  10. G Duran

    Bill Hilton Guest

    John Sheehy writes ...
    I don't believe this, based on what I've seen with Canon dSLRs. Can
    you post an example showing "orders of magnitude" improvement (what
    kind of camera are you using that's noisy at ISO 100?). Please name
    the camera and the RAW converter you're using and explain how you are
    doing this test too.

    Thanks.

    Bill
     
    Bill Hilton, Mar 20, 2005
    #10
  11. G Duran

    JPS Guest

    In message <>,
    I don't know about anyone claiming "OoM" differences, except in the
    realm of extreme under-exposure (like the 1600 vs 100 @ -4EC sample I
    showed). Comparing lower ISOs, especially normal exposure vs high, is
    less drastic.
    Your experiments are only measuring noise. They don't have any
    opportunity to be affected by signal quantization, which is finer when
    you shoot at higher ISOs and the absolute sensor exposure remains the
    same.
    This is true, and this is one reason why experiments like yours are
    difficult; you are using tools that assume that the brightest highlights
    are specular highlights, and that no-one would "overexpose" on purpose,
    which is totally wrong. Unfortunately, current RAW converters are
    insufficient; they should all have tonal transfer curves, etc, but they
    should also have pure linear pre-gain, and most don't. ACR, for
    example, will take RAW values above a certain point and nail them to 255
    in the output, no matter how much -EC you use.

    When I shoot RAW, I often envision better software down the road. Much
    of the noise in some cameras is patterned, like the bands in some dark
    20D pictures. Most if not all of the current RAW converters cause
    banding, because the readout offsets are different for each line, and
    they use a global offset. I've already seen the patterns disappear
    completely in DCRAW doctored to offset each line individually; the
    problem is most people don't want to use DCRAW; they want the GUI
    features of ACR and Capture One, which still use global blackpoint
    offsets.
    Well, part of the skill in "exposing to the right" is knowing how much
    you can. You have to assess the scene, and decide how much.
    --
     
    JPS, Mar 20, 2005
    #11
  12. G Duran

    JPS Guest

    In message <>,
    You misunderstand ... Sensor and readout noises are not the issue with
    very low ISO 100 exposures. The issue is *Posterization*, which can
    distort the image much more than sensor/readout noise.
    I don't trust the converters; I look directly at the RAW data. Canon
    10D and 20D are the cameras.

    --
     
    JPS, Mar 20, 2005
    #12
  13. G Duran

    Yip Yap Guest

    Perhaps you could can provide us with a little background
    here. Where did you come up with all of this information?
    Did you get it from Canon?

    -- Yip
     
    Yip Yap, Mar 21, 2005
    #13
  14. G Duran

    JPS Guest

    In message <zto%d.26288$>,
    This info is available by looking directly at the RAW data. A 10D ISO
    "1600" or "3200" shot has RAW data that looks like this:

    EEEEEEEEEEOEEEEEEEEEEEEEEEEEEEEEEEEOEEEEEEEEEE
    EEEEEEEEEEEEEEEEEEEEEEEOEEEEEEEEEEEEEEEEEEEEEE
    EOEOEOEOEOEOEOEOOOEOEOEOEOEOEOEOEOEOEOEOEOEOEO
    EOEOEOEOEOEOEOEOEOEOEOEOEOEOEOEOEOEEEOEOEOEOEO
    EOEOEOEOEOEOEOEOEOEOEOEOEOEOEOOOEOEOEOEOEOEOEO
    OOOOOOOOOOOOOOOOOOOEOOOOOOOOOOOOOOOOOOOOOOOOOO
    OOOOOOOEOOOOOOOOOOOOOOOOEOOOOEOOOOOOOOOOOOOOOO
    EOEOEOEOEOEOOOEOEOEOEOEOEOEOEOEEEOEOEOEOEOEOEO

    Here's an example from a real image, where the bitmap only shows the
    least significant bit (the one that determines odd or even):

    http://www.pbase.com/jps_photo/image/38841732/original

    The 20D is like this at ISO "3200":

    EEEEEEEEEEEEEOEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
    EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEOEEEE
    EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
    EEEEEEEEOEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
    EEEEEEEEEEEEEEEEEEEEEEOEEEEEEEEEEOEEEEEEEEEEEEEEEEE
    EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
    EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
    EEEEEEEEEEEEEEEEOEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
    EEEEEEEEEEEEEEEEEEEEEEEEOEEEEEEEEEEEEEEEEEEEEEEEEEE
    EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE

    What possible logical conclusion can one come too, except that these
    numbers were originally one bit less?
    --
     
    JPS, Mar 21, 2005
    #14
  15. G Duran

    Confused Guest

    The 20D programmers were chanting at the time?

    [ducks back under table]

    Jeff
     
    Confused, Mar 21, 2005
    #15
  16. G Duran

    Zed C. Pobre Guest

    This is really, really interesting. I'll have to play with this a
    little. ISO3200 is visibly noisier than ISO3200 level-adjusted from
    RAW, though, and I don't know why that would be if it's basically the
    same mode. If anything, the interpolation should smooth things, no?

    You mean a +1 EC? Or you mean playing with the EC in
    post-production by one stop? If you mean going for +1 EC on the
    camera, the extra light has to come from somewhere, either from
    aperture or shutter speed, and you wouldn't be playing with even
    ISO1600 unless those options were already blocked off. In that case,
    your option is between shooting ISO1600 one or more shots underexposed
    (thus wasting the extra RAW data space in the highest stop), or
    forcing interpolation in the camera to move data out of the lower
    stops.

    This is true, but when half of those numbers are in the top stop
    alone, you want to force your data over there if possible.
     
    Zed C. Pobre, Mar 22, 2005
    #16
  17. G Duran

    JPS Guest

    In message <>,
    I'm not sure what you're talking about here, or if you understood what I
    wrote. Let me restate it ...

    When the 20D is set to "ISO 3200" the metering system works as expected;
    it meters for ISO 3200. When the data is read off of the sensor,
    however, the same gain is applied as is applied for ISO 1600, and then
    the numbers are doubled after digitization. 0 stays 0, 1 becomes 2, 2
    becomes 4, 3 becomes 6, ... 2047 becomes 4094, and anything higher than
    2048 becomes 4095. Also, any mapped out pixels, interpolated from
    nearby pixels, have a 50% chance of being odd, as they are interpolated
    after the doubling.
    I mean -1 EC, and ISO 1600, in the camera. The midtones and shadows
    need +1 EC in the RAW converter, or post-processing The highlights can
    be clipped or squished at will.
    No, that wasn't what I was talking about. Setting the 20D to ISO 1600
    and +1 EC is basically "shooting at" ISO 800, with the top of the
    highlights clipped one stop lower than if the camera were set to ISO 800
    and ) EC, but with one bit more depth in the range captured; IOW, it's
    like having ISO 800, but instead of 0 to 4095, having 0, 0.5, 1, 1.5,
    .... 2047, 2047.5.
    That's called shooting at ISO 1600 and 0 EC. You shoot at ISO 3200, or
    1600 with -1 EC, because of lower light (or the need to stop action, or
    increase DOF).
    --
     
    JPS, Mar 22, 2005
    #17
  18. G Duran

    JPS Guest

    In message <>,
    Unfortunately, most camera manufacturers don't seem to care if you know
    how the RAW data is doing, even though that's really the most important
    aspect of exposure (quality-wise) at any given ISO. The only way I know
    is to get a program like IRIS that lets you look directly at the RAW
    data, and see what RAW values are present and compare them to the
    histogram and/or the JPEG, so you can get an idea of how much headroom
    there is. The only cameras I know well in this regard are the Canon 10D
    and 20D; the 10D starts to blink or clip the histogram when the green
    channel is about 1 stop from the green RAW highlight limit. The 20D is
    more like a 1/2 stop, as it stuffs an extra 1/2 stop into the top of the
    8-bit output (about 245 to 255).
    --
     
    JPS, Mar 23, 2005
    #18
  19. G Duran

    Zed C. Pobre Guest

    Well, partly, I meant to write "visibly noisier than ISO1600", and
    partly, I may just be very confused.

    I'm with you so far. What I'm trying to work out in my head is
    where the extra noise is coming from at ISO3200 (I thought it was
    related to gain), and whether or not there's any advantage in terms of
    dynamic range to shoot at ISO3200 than at ISO1600 (if I'm
    understanding you correctly, it seems that the answer is no for RAW,
    and the only time you'd ever want to use ISO3200 is when you're
    shooting straight to JPG).

    Okay, either I have a sign backwards, or I'm not at all
    understanding what you're doing here. Just to be absolutely clear,
    when you say "-1 EC", you mean "underexpose by one stop" (relative to
    what the camera light meter thinks is appropriate exposure), correct?
    This combined with ISO1600 is two stops darker than you would get by
    default with ISO3200, squishing everything far to the left in a scene
    that is dark to begin with. I'm not understanding why you would want
    to do this.

    If you mean "overexpose by one stop", then this makes more sense,
    but the sign seems to be contrary to what I see in the camera and in
    the EXIF data in the saved files.
     
    Zed C. Pobre, Mar 23, 2005
    #19
  20. G Duran

    JPS Guest

    In message <>,
    We're on different trains of thought.
    Noise comes mainly from the signal-to-noise ratio. The noise in the
    sensor is the same at all ISOs. The main reason that the same scene
    auto-exposed at ISO 1600 is noisier than the same scene at ISO 100 is
    that the signal is a lot weaker at ISO 1600, and the noise is
    consequently higher in the ratio. There is also some extra noise and
    distortion from the amplifier, that becomes a bit of a factor at the
    highest ISOs.
    Well, another reason is if you want to shoot 1600 with auto-exposure,
    but three stops under, the only way to emulate it in auto mode is with
    3200 and -2 EV, since the 20D only has EC down to -2.
    It's one stop darker, in the JPEG, or default RAW conversion. I don't
    know where you're getting the "2" from. In the actual sensor exposure,
    they are exactly the same (but the digitization will only digitize half
    the dynamic range with "ISO 3200" that it does with ISO 1600 and -1 EC).
    Because you don't have a choice, a lot of times. Even if you set the
    camera to ISO 1600 and PLUS 2 EC in shutter priority mode, if there
    isn't enough light the exposure is going to be negative anyway, even
    thought the camera wasn't set that way, Many fast lenses also have
    horrible optics wide open, such as the Canon 50mm f/1.4 and 24mm f/1.4L.
    Just stopping down to f/2 can cause a drastic improvement in corner
    optics, so setting the camera to shoot a little on the dark side to
    avoid 1.4 as much as possible is an option.
    --
     
    JPS, Mar 23, 2005
    #20
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