ISO and actual sensitivity in DSLR's (D70, *istD, 20D, S3...)

Discussion in 'Digital Photography' started by Alan Browne, Mar 25, 2005.

  1. Alan Browne

    Owamanga Guest

    It is entirely separated from the compression (file coding) used to
    store the image.

    Fill a 100x100 pixel image with 118/255 RGB in photoshop and save it
    as a JPEG, BMP, TIFF, PSD even GIF, choose any of the formats, lossy
    or lossless. Now open the file again, and you'll see data that shows
    Agreed, that wasn't my point. I was trying to explain the way Alan was
    measuring - eg the meaning of 118 is 118/255 in photoshop's pixel
    level 'info' tool.
    Agreed, a good idea on any camera. But Alan isn't attempting to
    measure DMin or DMax here, so the blown highlights isn't really going
    to be an issue.
    Yes. There is more than one way to cook an egg.
    Your gamma settings will make no difference to the info tool's ability
    to point to a pixel and give you a reading of 118/255. All it would do
    is make that gray appear to be darker or lighter than another system
    displays it. Alan wasn't metering off his screen, so PS's color
    management doesn't really come in to it.
    Presumably the color space consistently remains sRGB from the camera
    to Photoshop. Or if a color space change was involved, it was the
    *same* color space change from each source.
    Okay, but to compare two sources, when Photoshop's configuration
    remains the same between the two, I think still has some validity.
    Owamanga, Mar 28, 2005
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  2. Alan Browne

    Alan Browne Guest

    Try READING the post. I've stated several times that the Adobe RAW
    import utility in 16 bit/colr mode displays the R,G,B data in the range
    of 0..255. Hence, 118 / 255 is the 18% grey level.

    I also posted a link to a screenshot: whiuch you seem to have
    ignored for the sake of being a difficult curmedgeon.
    Been there, done that. However the method use dby C d'I is different
    (yet in reference to the same standards and processes). The method I
    used results in a similar outcome.

    Reading for middle grey is a legitiamte means of confirming the ISO of
    the cameras. They all have different toe and shoulders, but they should
    all have the same 18% grey if their ISO sensitivity is correct.
    Alan Browne, Mar 28, 2005
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  3. Alan Browne

    Alan Browne Guest

    It is as pure as the driven snow as it completely ignores display and
    conversion issues. It is not JPEG. It is not TIF. It is not PSD. It
    is not anything other than what the sensor read at a specified color
    temperature. Period.

    Thence, converting to JPG without any other changes, it is not
    surprising that the JPG shows pretty much the same number (within a few).

    If this does not satisfy you, then please explain the reverse: Why am I
    getting these numbers? Coincidence perhaps? No.
    I was not using the light meter.
    I was using RAW to record.
    Perhaps not to define it. However, if 18% grey is what it is supposed
    to be, then a shot taken of such an 18% target under conditions that
    eliminate as many uncertainties as possible (as I did) will reveal if
    the sensor exposed for a particular ISO is recording for that ISO. In
    the 7D case it is thus. In the A200 test (C d'I) it is thus.
    Before. It is completely independant of colorspace. It is pre-color
    management (display/printer).

    You're steadfastly missing the point. The 18% grey test is not to
    DEFINE the ISO, but rahter to confirm the exposures taken with a given
    IOS setting. If the 18% comes out as 18% then the ISO is correct in the
    mid tone. Otherwise, as people use their meters to measure light
    relative to an 18% mid tone, they will end up with over or under
    expsosure of the mid tone.
    Alan Browne, Mar 28, 2005
  4. The camera might output Adobe RGB. The screen is probably sRGB.
    The working color space inside Photoshop can be either or something
    totally different. The 118/255 is meassured within the working color

    Moreover - even if the working color space and the camera's color
    space are equal - the picture has to be converted from the linear
    representation to the non linear 8 bit representation before it
    can be representaed as 118/255.
    Yes - it has some validity. But only as long as you plan
    on using the same conversion utility when you are going to
    make the pictures as you used when you did the measurements.

    Roland Karlsson, Mar 28, 2005
  5. Alan Browne

    Alan Browne Guest

    How many times have I stated: RAW for my tests?
    Again, try reading. I stated "in the Adobe RAW converter."

    Yes and no. First off I am a 100% RAW man (since I finally found out
    that PS E 3.0 does 16 bits/color). My images will always come in to PS
    via that path.

    No, in the sense that once the ISO offset or error is known, then the
    photog can make further tests to determine his own exposure preferences
    for his work path.
    Alan Browne, Mar 28, 2005
  6. OK - your test meassures the fidelity of mapping an 18%
    grey card to the value 118 in the controlled environment
    and using the given work flow.

    But - personally I fail to se the relevance of such a test.
    At least I always apply levels to my pictures. Then the 118
    value will move. I also apply some color balance. Then it will
    move even further.

    The main purpose of the choice of exposure is to keep the
    important parts of the picture within e.g. 10-90% of max
    exposure before clipping. Where the 18% grey is to be found
    in an 8 bit conversion of the data I find rather uninteresting.
    If the picture is rather flat I over expose some to move
    the exposure to parts with less noise - or under expose some
    to avoid blurriness.

    Roland Karlsson, Mar 28, 2005
  7. The 118 cannot be a linear value. It must be a predicted value
    by the Adobe RAW utility. And this is exactly how Photoshop
    displays the values whan working with 16 bit pictures. Photoshop
    (and I assume Adobe RAW) shows the predicted value in the assumed
    working color space using 8 bit representation. This is a non
    linear mapping of the 16 bit linear data. And this non linear
    prediction might vary between conversion utilities.
    I have looked at it. And I cannot see that it changes a thing.

    Roland Karlsson, Mar 28, 2005
  8. Alan Browne

    Alan Browne Guest

    Explain then the quite improbable coincidence, given all your tripe,
    that the results I get are so on the money?
    Alan Browne, Mar 28, 2005
  9. Alan Browne

    Owamanga Guest

    To summarize, a correct exposure isn't important in your workflow
    because you have the levels sliders in Photoshop.

    It all makes sense now.
    Owamanga, Mar 28, 2005
  10. Yes you have - but repeating it does not make it
    more valid :)

    This is what really happens when you read 118.

    I am 100% sure about it. If you have some arguments
    I find valid - telling hat I am wrong I will of
    course listen. But don't repeat that you are working
    with RAW data - as I find that remark being irrelevant.

    The 118 value is _not_ taken from the RAW data, it is
    _computed_ from the RAW data. The Adobe RAW conversion
    utility does exactly like Photoshop when working with
    linear input. Photoshop maps the 16 bit PSD file to
    the choosen working color space. This color space might
    be sRGB and is almost always non linear. It might also be
    Adobe RGB. You can check what you have choosen in the
    Adobe RGB utility. It is not a surprise that the converted
    file has the same values - as the Adobe RAW conversion
    utility uses the same computation when doing the conversion
    as it uses when computing the predicted value.

    So - if Photoshop tells me 118/255 it then means the
    value that the 16 bit linear pixel gets when it is converted
    to the working color space represented as 8 bit.

    If you take a linear 16 bit PSD picture and work in another
    color space - then the 118 will be displayed as something

    The 118 does not say anything about the greyness of the pixel.
    It only tells what the pixel will be represented as when
    saved in an 8 bit file using the working color space.

    Roland Karlsson, Mar 28, 2005
  11. I have written a long reply further down that explains what
    happens when the Adobe RAW utility shows 118. I hope this
    is the explanation you seeks.

    Roland Karlsson, Mar 28, 2005
  12. Matte white objects like cloth and paper are about 90% reflectance. 18%
    grey is 1/5 of that. So, *if* the camera exposed to put white at 4095,
    you'd expect a grey card to be at about 820. Mid-grey is "mid" to our
    eyes, but we see in an approximately logarithmic space, not a linear

    In practice, a camera isn't likely to put white at 4095 if it's setting
    exposure automatically, because it wants some headroom for capturing
    things brighter than white (e.g. specular reflections) without clipping
    them abruptly. So white might be at 2048, grey at 410, and you'd have 1
    stop of headroom above white.

    Dave Martindale, Mar 28, 2005
  13. Well, no. The sensor read some voltage, which was probably converted
    to a number in a range of 0-4095 or 0-16383 or something similar. Then
    Photoshop's raw converter mapped that to the non-linearly coded (gamma
    corrected) value 118 out of 255. 118 isn't raw data, and it also can't
    be related back to an original sensor measurement without knowing what
    the Adobe raw converter does internally.

    And if the raw converter does any part of its processing adaptively,
    depending on image content, you can never know exactly what the original
    data was - even with a knowledge of internal operation of the converter.

    Dave Martindale, Mar 28, 2005
  14. Alan Browne

    Alan Browne Guest

    <sigh> As stated before all the data I posted was with all of the RAW
    conversion parameters set to 0 with the exception of color temp. That
    is to say no effect on the image. If I change parameters, such as
    brightness, contrast, aturation, etc. you can be sure that the info
    values change accordingly.

    From the 7D manual: "Unlike the other image-quality modes, RAW image
    data is unprocessed and requires image processing before it can be
    used". Since the 'processing' I've done is _none_, there is no effect.

    Secondly, due to the neutral conversion the JPG shows the same values
    (close enough) at the same points in the image.
    Alan Browne, Mar 28, 2005
  15. Alan Browne

    Alan Browne Guest

    1) From the 7D manual
    "Unlike the other image-quality modes, RAW image data is unprocessed and
    requires image processing before it can be used"

    2) As prev. stated, I had all 'adjustments' set to 0. Only the color
    temp to match the light source was used.
    Alan Browne, Mar 28, 2005
  16. In a sense you are right. If you use your digital
    camera (expensive or not) and takes a photo then
    the most important thing is that the important
    parts of the picture is within 10-90% of the exposure
    range. Then you can make a picture that prints well.

    In a sense you are not. It is important to
    use a consitent exposure in your work flow. At least
    if you are doing studio work. Pre visualisation
    when taking the picture combined with a repeatable
    work flow gives you better pictures.

    But - I don't think that the meassurement where
    you map 18% grey in an evenly lit scene to 118
    in a sRGB picture is a useful method to get a
    consistent work flow. I think it is a misunderstanding
    of how exposure and color spaces work in digital

    Roland Karlsson, Mar 28, 2005
  17. Please Alan.

    Reread what I wrote. There is no 118 in the RAW data, the value
    118 is computed. And it is the same computation made when
    converting to JPEG. The 118 is the result of a non linear

    That Adobe RAW says that it is working with the actual RAW
    data is also correct. But it has nothing to do with the value
    118, because that value is not what Adobe mwans whith image

    You can <sigh> how much you want. If you think I am a problem
    when I am right and you don't understand - then it will soon
    be your problem only - when I give up to explain it to you.

    Roland Karlsson, Mar 28, 2005
  18. Alan Browne

    Alan Browne Guest

    I agree to this point, I was a bit heavy in stating "the senor" above.
    It should be, as stated in the 7D manual: "Unlike the other mage-quality
    modes, RAW image data is unprocessed and requires image processing
    before it can be used". It is such when I look at the pixel values,
    except for the light source temp.
    At the RAW converter stage, I'm looking at the numbers for a given pixel
    converted to the 0..255 scale, not the post conversion image/data where
    gamma has been applied. There is, to put a point on it, no gamma
    coefficent settable in the RAW converter and I have all settings set to 0.
    Agree up to the point that I have the converter set (for this test) at
    all '0', only the light source temperature is set.

    If everyone else is correct about this, then I would like someone to
    elaborate on the extraordinary coincidence that my values were very
    close to 118 (except blue which was a little bit lower).

    Alan Browne, Mar 28, 2005
  19. Alan Browne

    Alan Browne Guest

    For tour own reasons you've completely misdirected the purpose of the OP.
    Alan Browne, Mar 28, 2005
  20. I hope you can bear with me - although you start to get
    annoyed :)

    If we assume that neither the camera nor the conversion
    utilty finds any reason why it should do any compensations
    and we also assume that the lens is without much flare and
    we also assume that the non linear color space conversion
    is well known (e.g. sRGB or Adobe RGB - well implemented) -
    then a correctly exposed evenly lit 18% grey card shall
    have a specific value in the picture.

    I assume someone else have made similar meassurements like
    you and found out that this value is 118. So - if you do the
    same meassurements thoroughly you shall end up with 118.

    But - and this is my point. Another camera or conversion
    utility might end up with 132. And this value do not show
    that the camera/utility has another sensitivity. It only
    shows that the value is 132. To understand the sensitivity
    of the acmera you must look at more values. And most important
    are those near to 255 and 0.

    In a way it seems like you want to use something like
    the zone system. In the zone system, the picture is matched
    into 10 zones in the print. But ... you are trying to
    use only one zone.

    Roland Karlsson, Mar 28, 2005
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