RAW and ISO settings

Discussion in 'Digital SLR' started by lemax, Jul 11, 2005.

  1. lemax

    lemax Guest

    Does anyone understand what are the mechanics behind the ISO settings
    for RAW images?

    lemax, Jul 11, 2005
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  2. lemax

    eawckyegcy Guest

    Yes, I understand them. At least for the Canon 1D Mark II: there are
    three buttons, a finger wheel, and a thumb wheel. Maybe the Nikon D70s
    has the Telepathic Adaptor which can lock onto the mind of the
    photographer, thus dispensing with these crude mechanical implements?
    eawckyegcy, Jul 11, 2005
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  3. lemax

    McLeod Guest

    I think the poster was asking a more generalized question than you
    responded to, but hey, who am I to stop you from making a jerk of

    Usually the lowest ISO setting of your digital camera is going to be
    where you get the best picture with the lowest noise. Higher settings
    are amplified, I believe, so noise is increased.

    They are only in increments of 100, 200, 400 to remain reciprocal with
    shutter speed and aperture settings.
    McLeod, Jul 11, 2005
  4. lemax

    Frank ess Guest

    I think the OP was asking how changing the ISO setting affects how the
    camera responds to a light stream striking the sensor when the camera
    is set to record raw images.

    I don't know.
    Frank ess, Jul 11, 2005
  5. lemax

    eawckyegcy Guest

    You "think"?
    You "believe"?
    Well, you have offered two speculations and one error of fact. And you
    accuse me of making a 'jerk of myself' for correctly answering his
    question as written? Nitwit.
    eawckyegcy, Jul 11, 2005
  6. lemax

    McLeod Guest

    And yet I have still contributed more than you. Unless you consider
    your original answer serious.
    McLeod, Jul 11, 2005
  7. lemax

    eawckyegcy Guest

    Only a dingbat can consider an error a 'contribution'.
    The OP asks if "anyone" "understands" the "mechanics" of ISO settings
    and RAW images.

    I answered, correctly, that I do for my particular camera. I outlined
    the answer; I can elaborate if anyone wishes. Now if the OP had
    _another_ question in mind, he is certainly free to post it.

    Now then, what _is_ your excuse? Or are you just being deliberately
    wonky for entertainment purposes?
    eawckyegcy, Jul 12, 2005
  8. lemax

    lemax Guest

    Sorry, I realize now that the original message was not "descriptive"

    How ISO works on film is rather "material" - the grain of ISO 400
    compared to let's say 50, has a different, more corse structure.

    But how does it work for RAW in digital storage? "Approximation"
    algorithms are not supposed to be applied and whatever light reaches
    it should be recorded no matter what ISO is.

    Any ideas?
    lemax, Jul 12, 2005
  9. lemax

    DoN. Nichols Guest

    It depends on the camera. The Nikon D70 has an "AUTO-ISO" mode
    (selected from the menu), in which, first the camera attempts to select
    an appropriate shutter speed and aperture at the base ISO of 200, and if
    that does not achieve the desired effect, it then increments the ISO
    until a "reasonable" combination of shutter speed and aperture are
    possible -- or until it gives up at the top ISO.

    Menu entry 21 allows you to select a "floor" shutter speed, from
    1/60 of a second down to 30 seconds, depending on what your needs are.
    Yes, a higher floor would be nice with longer lenses, but with longer
    focal lengths you are somewhat more likely to be using a Nikon VR lens
    ("Vibration Reduction" the equivalent of Cannon's IS "Image
    Here, again, this is camera dependent. It may be so with your
    Cannons, though I don't know, as I've not used one. However, the Nikon
    D70 allows ISOs in the following steps:

    200, 250, 320
    400, 500, 640
    800, 1000, 1250

    Rather finer steps than those which you listed, but the D70
    adjusts exposures in 1/3 stop increments.

    As for the mechanism of the actual ISO changes, it is
    accomplished by one or more variable gain stages between the analog
    sensor cells and the A/D converter(s) which feed the digital signals
    into the camera's buffer memory. I don't know the count of A/D
    converters, but I would expect multiple ones, each allocated to a
    subgroup of sensors, so the A/D conversion could be accoplished at least
    partially in parallel, to speed up the process, and thus to clear the
    camera's sensor for the next shot more quickly.

    DoN. Nichols, Jul 12, 2005
  10. lemax

    Jeremy Nixon Guest

    The signal is amplified to reach the desired level.
    Jeremy Nixon, Jul 12, 2005
  11. lemax

    MrB Guest

    His reply has nothing to do with your rude reply to the original poster.
    Your reply was still RUDE.
    MrB, Jul 12, 2005
  12. lemax

    lemax Guest

    Great. The last paragraph is what I was looking for.

    Thank you, DoN
    lemax, Jul 12, 2005
  13. lemax

    lemax Guest

    Got it now.

    Thank you.
    lemax, Jul 12, 2005
  14. lemax

    DoN. Nichols Guest

    As I just got through typing (and thus, you have not yet had
    time to read it), and perhaps I can add a bit more detail this time, now
    that I know the focus of your question.

    1) The sensor cells (CCD or CMOS) have capacitors which store a
    charge during the exposure. The maximum change which can be
    stored defines the low-end ISO for the sensor, as more exposure
    fills the cell to the maximum, and highlight detail is lost.

    2) Once the exposure is done, the charge is moved from the sensor
    cells towards circuitry on the border of the sensor.

    3) The first bit of circuitry will be a gain stage, so if the
    exposure was not sufficient to fill *any* of the cells, the
    range of voltage resulting from the charges which *were* stored
    is increased by applying amplification, to provide a reasonable
    range of voltage to the next stage.

    4) The second bit of circuitry will be the A/D converter (Analog to
    Digital converter.) This converts the analog information from
    the sensor cell which came through the amplifier into a number
    which represents that level in a digital form

    Note that there is always *some* noise in the sensors, though it
    may be small enough so it is not noticed in the presence of the signal
    at low gains.

    However, if the gain is high, it is amplifying both the noise
    (which is approximately a constant level for a given exposure time) and
    the signal. This brings the noise up to a level where it is obvious
    (typically on darker areas of the photograph).

    There are at least three sources of noise:

    1) "Thermal" noise, which is a function of the resistance
    (impedance) that the sensor and circuitry up to the amplifier
    sections and the temperature. The higher the temperature, the
    more thermal noise, and the higher the gain (ISO) the more
    visible the noise from a given temperature will be. Far
    Infrared sensors are often operated at very cold temperatures,
    such as 70K (the temperature of liquid nitrogen). You've
    probably seen far IR images in the video from missile strikes
    from aircraft from the Gulf War footage.

    2) The digital "clocking" noise (which is left-overs from the signals
    used to move the information from the sensors to the A/D

    3) "Fixed Pattern" noise, resulting from minute variations in the
    sizes of the sensors, and slight variations in the accuracy of
    the A/D converters.

    This last one has the advantage of being somewhat predictable,
    and many cameras have a mechanism to subtract this out from the
    image data. This is commonly done (for long exposures) by
    closing the shutter, and collecting noise for the same time as
    the exposure just collected, and then digitally subtracting the
    noise from the signal data.

    The noise from these various sources all tends to be worse for
    long exposures or high ISO, and as a result, it behaves in a manner
    somewhat similar to film pushed to high ISOs. However, the film won't
    have the Fixed Pattern or the clocking noise sources, just the
    equivalent of the thermal noise. (There is a difference. As you push
    the ISO in a digital sensor, the brightness variation becomes greater,
    but the size remains that of a single pixel. As you push the ISO in
    film, however, individual grains become much larger than the pixel size
    from the digital image at the same ISO, so the noise (grain) from
    seriously pushed film is more obvious than that from the sensors at
    higher ISOs. Yes, you can see the noise when you push a Nikon D70 to
    1600 ISO, but it is not nearly as obnoxious as the grain in film pushed
    to the same speed.

    Also, in digital images, there are various algorithms to reduce
    the noise. Some are built into the cameras (including that for
    correcting fixed-pattern noise), while other may be in the camera, or
    may be in the computer during post-processing.

    The D70 is not particularly aggressive in in-camera noise
    reduction processing, and (from what I have read) the Cannon is much
    more aggressive. One side effect is that the Cannon (at least the
    Digital Rebel cameras) appears to produce softer images, as sharpness is
    one of the casualties of aggressive noise reduction. I am rather glad
    that the Nikon leaves it to post-processing (in the computer), so I can
    make my own choices between sharpness and noise.

    I'm not sure how much of this noise reduction may be done in a
    Cannon camera in RAW mode.

    I do know that this last 4th of July, I opted to shoot with the
    in-camera long exposure noise reduction turned off, so I was not stuck
    waiting for the same time as the last exposure to gather the noise
    reduction information before I could shoot again. For most images, it
    came out rather nice, but for some (when I held the shutter open for
    longer than I expected, thanks to delays in the launching of the next
    fireworks), I wound up with serious fixed-pattern noise in those images.

    To see some selected examples, visit the following URL:


    I hope that this helps,
    DoN. Nichols, Jul 12, 2005
  15. DoN. Nichols wrote:
    Don, thanks for this, but you have missed one important noise contribution
    to the final image, which is photon-limited noise. In a nutshell, this is
    a characteristic of light which produces an uncertainly in the number of
    photons according to the square root of the number of photons collected.
    So if the sensor well can only hold 10,000 electrons, there will be an
    uncertainty of 100 electrons, and hence a best possible "signal-to-noise
    ratio" of 100:1 at each pixel. Hence the desire for a large well size to
    maximise SNR, and hence larger area sensors, and lower ISO settings.

    David J Taylor, Jul 12, 2005
  16. lemax

    DoN. Nichols Guest

    [ ... ]

    [ ... ]

    [ ... ]
    You have a good point -- though I'm not sure that I would class
    this with "noise". It is at least an "uncertainty". And the fewer the
    electrons, the more effect the uncertainty has.

    DoN. Nichols, Jul 13, 2005
  17. lemax

    lemax Guest

    The information you gave is far beyond I would've been able to gather
    myself. The level of your understanding is amazing and you have my
    greatest appreciation for the willingless to share.

    Thank you all.

    lemax, Jul 13, 2005
  18. lemax

    DoN. Nichols Guest

    [ ... all snipped at this point ... ]
    Well ... it helps that I used to work with IR the development
    and testing of IR imaging sensors, so I had plenty of time to absorb the
    information. It was just a matter of re-describing them in terms of
    visible light cameras.
    You're welcome,
    DoN. Nichols, Jul 13, 2005
  19. DoN, it's noise in the sense that if you photograph a uniform field, the
    value of each pixel within the field will not be a single value, but will
    have the photon-noise uncertainty superimposed on top of the nominal
    value. Thus is looks just like the other noise sources you describe.

    I can't remember now if the statistics of the noise are Gaussian (Poisson
    rings a bell), and whether the square root is the RMS value of the noise.
    I also used to work with IR sensors, by the way, cooled to 77K as I
    recall. I modelled the entire vision system - target, atmosphere, sensor,
    monitor, eyeball, brain.

    David J Taylor, Jul 13, 2005
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