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

    Alan Browne Guest

    I just repeated the test as follows:

    0. Shot grey card at f/5.6 using strobe set at f/5.6 incident metered
    (for ISO 100) with K-M 7D.

    1. RAW import of the same shot.

    2. In RAW import (Adobe) set all sliders to 0; temp to 5500K.

    3. Within the RAW import, 8-bit end of R,G,B = 119,117,105 (typical,
    many values around there with the blue being consistently 10 or 12
    levels below the R and G. (Whether this is a reflection <npi> of the
    card or the sensors is not determined).

    4. Imported that into E 3.0. R,G,B values are the same.

    5. Converted to 8-bit and saved as JPG

    6. Reopened. Values are as above.

    Alan Browne, Mar 26, 2005
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  2. If the D70 varies in sensitivity per individual camera then the D70
    would be irrelavent. In fact, I am sure that they are consistant
    between cameras and as such, it this is all useful information to have.
    I own the D70 and will likely be working with film (Nikon N80) in the
    not too distant future and it will be useful to know that my experience
    with ISO settings on the D70 are likely to require a factor of 0.8 when
    going to film.
    Thomas T. Veldhouse, Mar 26, 2005
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  3. Hmmm ... how can that be? The sensitivity of a sensor
    is defined by the saturation level.

    The conversion to JPEG depends on color space and the
    bias you use for representing 255. So - the mid tone
    can (IMHO) be anything in the JPEG.

    Moreover - the R, G and B are only equal if the system
    is in perfct color balance. And that is highly unlikely.

    Roland Karlsson, Mar 26, 2005
  4. Alan Browne

    Alan Browne Guest

    ....then there's the meters...

    Alan Browne, Mar 26, 2005
  5. Alan Browne

    Alan Browne Guest

    We've gone through this before. As the parameter being tested is
    fidelity to the ISO setting, the mid tone is the only place the can be
    measured. The toe and shoulder should vary too much between camera
    models to be of any use in comparison.

    As my own test shows, the three are not in perfect balance, and one
    should not expect or hope that they would be. Further, and as
    mentioned, and again I suggest you get a copy of the mag, the ISO table
    they published and that I repeated are rounded numbers.


    Alan Browne, Mar 26, 2005
  6. Hmmm ... I assure you ... I am not trying to be difficult.
    But ... I have a hard time to believe in this. I have added
    some comments below.
    I am sorry - I don't understand this at all.
    Nor do I understand this. This might be because I have
    no knowledge about Adobe RAW import. It looks like step
    3 and 4 makes some modifications of the RAW data upon import.
    How can we use the data for meassurements then? The data shold
    be unaltered IMHO.
    Converting to 8 bit can be made in lots of ways. gamma?
    color space? bias? This is another step where the data
    is modified and therefore makes meassuremets invalid IMHO.
    To mee it looks like the correct method is to compare
    the level of white clipping and then use the definition
    for ISO sensitivity for sensors.

    How the image looks after converting with Adobe RAW to
    JPEG seems totally bogus to me. If you don't want to
    test the workflow described above of course. But most
    serious photographers do not convert to JPEG.
    Roland Karlsson, Mar 26, 2005
  7. Hmmm .. after some thought I withdraw this suggestion.
    The clipping level might be hard to find.

    The problem is even more complex really. There are two
    different things you can meassure.

    1. Given a standard luminosity of the picture - what
    exposure time and aperture do the camera choose.

    2. Given a standard luminosity of the picture - what
    is the exposure on the CCD with a given exposure
    time and aperture.

    Case 1 meassures the light meassuring bias in the camera.

    Case 2 meassures the light sensitivity of the system.

    Both are two interesting results. But they are different.

    Roland Karlsson, Mar 26, 2005
  8. ISO 12232 should tell the whole story. But I'm not going to pay $62
    for it.

    The following link is quite interesting:

    "There are two basic types of ISO: saturation-based and noise-based.
    "The saturation-based ISO is also referred to as the `base ISO.'
    "The type of ISO of interest depends on the application at hand. In
    "applications where the lighting conditions are controlled, like
    "studio photography, exposure index settings are selected to give
    "the best possible image, with the image highlights falling just
    "below the saturation level. This exposure situation is described
    "by the saturation-based ISO.
    "When lower that ideal lighting conditions are expected, a noise-based
    "ISO is more useful. In this calculation the signal to noise ratio
    "(SNR) that gives a reasonable image for the application of interest
    "is used to calculate the ISO. This ISO corresponds to the maximum
    "EI setting to achieve that SNR. It is common in photography to
    "designate SNR=40 as an `excellent image' and SNR=10 as an `acceptable
    "image' and calculate a corresponding noise-based ISO for each.
    Philip Homburg, Mar 26, 2005
  9. SNIP
    Yes, ISO (to be exact ISO 12232) speed for digicams is well defined,
    I've summarized it (based on an earlier draft version of the standard)
    in an earlier thread:

    Bart van der Wolf, Mar 26, 2005
  10. Alan Browne

    Alan Browne Guest

    Hmmm yourself.
    Grey := Red = Blue = Green. When high values, pale grey, when low
    values, dark grey. 18% grey is 118/255.
    (A result of a coefficient from the ISO docs that is 0.461 of the
    maximum level. 0.461 X 255 = 117.56)

    The math is in the article so get down to your local newsstand and buy
    the edition.
    It is correct as long as all conversions are set to 0. (As it was).
    The only variable is the color temp of the light source which I set to
    that of the flash.

    By the way, this would be the same for film. Wrong color temp for the
    film would render results that are not useful.
    Gamma goes to dislay/print, not content. Using 'info' in PS to examine
    the content of a pixel is independant of gamma.
    Regardless of the conversions:

    -the grey level was maintained exactly if the RAW conversion was held to
    no changes other than color temperature. eg: the 8 bit values in the
    RAW and JPG were the same.

    -you can't make such a comparison at the shoulder or toe of a variety of
    sensors. You would get incomparable data.
    Alan Browne, Mar 27, 2005
  11. Alan Browne

    JPS Guest

    In message <d26dqa$5jl$>,
    What do you mean by "the 8 bit values in the RAW"?

    The exposure of a grey-card in the Canon 20D is in the 300s in the green
    channel, out of up to 3975 RAW levels above blackpoint.

    The whole idea of characterizing cameras that produce RAW files by their
    JPEG renders makes me cringe. JPEG is bullshit; it's just a side effect
    of the camera; an extra feature with limited value. It makes me sick
    that my $1500 DSLRs have green-channel-only histograms that represent a
    "JPEG", and tell me nothing about the state of the real exposure, as it
    is digitized in the RAW "RGB" data.
    JPS, Mar 27, 2005
  12. Alan Browne

    Alan Browne Guest

    In the RAW import utility of PS E 3, it displays R,G,B as 0..255 (using
    the info pointer) even if in 16/bit / color mode. Here's a screen shot. (the pointer was on the grey
    back of one of the gulls).

    4095 minus blackpoint = 3975 ?

    300 / 3975 = 0.075
    399 / 3975 = 0.1

    Seems well below 18% grey. But I suppose this is non-linear.
    First off, in my test of this, the R,G,B's are those reported in the
    Adobe RAW converter (set at 16 bit/channel, but the RGB info is
    displayed in the range 0..255 for each color).

    Regarding your "green channel only histos" I suggest you write a latter
    to the manufs. By the way, do you have a reference for it being
    green-channel only?

    The C d'I article did not state how they were getting the RGB info (what
    software), just that they were. When I imported the RAW into PS E 3., I
    had to set all conversion sliders to 0 (except color temp which I set at

    In PS, of course it's not JPG, it's native PS. The info window/pointer
    gives the values at that point.

    Alan Browne, Mar 27, 2005
  13. :)

    Nope - I don't buy it.

    The cameras we are talking about use RAW as their native
    format. Any meassurements on ISO sensitivity must be made
    on the original RAW data.

    The procedure you describe means that this RAW data is
    modified and then converted to JPEG before making the
    ISO sensitivity meassurements. This conversion involvs
    both color correction and also an unknown non linear
    gamma type conversion.

    The strobe method for exposure is also somewhat peculiar.
    This method avoids any inaccuracys in the light
    meassuring system of the camera and also any inaccuracies
    in the exposure time. But - you still have the accuracy
    in the aperture and lens attenuation to take into

    I would say that the method (although involved and
    rather scientific executed) is rather meaningless.

    Roland Karlsson, Mar 27, 2005
  14. There are three ISO values you can meassure for a camera.

    1. The ISO the light meter in the camera assumes.
    2. Saturation based ISO.
    3. Noise based ISO.

    All three can be found in this paper:

    To meassure 1. you use a large uniformly lit area. You then meassure
    this area with a callibrated light meter. Then you meassure the same
    with the camera's light meter. The actual value can then be deduced.
    E.g. if you set the camera to ISO 100 and the callibrated light meter
    gives you the same exposure at ISO 125 - then the camera actually
    is meassuring ISO 125.

    How you can meassure both 2. and 3. is hinted at in the Kodak paper.
    It is rather involved - and it does not look the slightest like
    the one you described was made in the journal.

    The three methods will probabaly give you three different values.
    And this is OK. You can expose more or less and still get a
    useful image. The actual ISO for the system depends on how you
    want to use it.

    Roland Karlsson, Mar 27, 2005
  15. Alan Browne

    Alan Browne Guest

    As I did. I assume that C d'I did as neutral a test as possible, but
    they did not state how they did that. They do state various things such
    as their dark room, their light source (2000 lux), etc.
    Again, the measurement _I_ did was off of the RAW first then JPG. With
    (as I stated) all parameters set to '0' for conversion to JPG, the
    values in grey remained about (+/- a few) the same. (Only color temp
    was set to flash temp of 5500K).

    I chose the flash as I could accurately meter it seperately, it's a very
    repeatable test, I could do with the studio lights down to avoid other
    color sources.

    As to the aperture I'm already satisfied from other experience that it
    is accurate enough.

    As to lens attenuation, that is also fairly insignificant. (Or why
    would incident metering be useful at all).
    I would say that their method and mine beat your conjecture.

    Alan Browne, Mar 27, 2005
  16. Alan Browne

    Jeremy Nixon Guest

    You may be misinterpreting what "linear" means.

    Assume for sake of simplification that the camera is using all 4096 levels
    to represent black to white. Level 2048, the halfway point, is now one stop
    down from maximum saturation, because one stop down is half as much light.
    Level 1024 is, then, two stops down, 512 is three stops, and 256 is four
    stops down.

    The 0-255 scale you're thinking in is non-linear in terms of the amount of
    light represented -- it represents levels after a gamma correction.
    Jeremy Nixon, Mar 28, 2005
  17. The values in RAW are 12 bit linear - the values in JPEG are 8 bit
    _non_ linear. How can you be meassuring the value 118 on the RAW data?
    That would be almost totally black. There must be some misunderstanding

    Moreover - look at my other reply that shows what ISOs you
    can meassure. There are actually several ones, depending on
    what you want to accomplish. And if you don't believe me or
    my argumentation - maybe you believe Kodak. The link I give
    in the other reply is to a Kodak document. Please read it.

    Roland Karlsson, Mar 28, 2005
  18. Alan Browne

    Owamanga Guest

    If you took a look at the binary file itself (after some
    decompression, and a good idea of JPEG format) this is true.
    He's keeping the same method of measurement across both formats, 118
    means 118/255 on each of the RGB channels. Photoshop has already
    interpreted the RAW file at this point and moved it into these 3 bytes
    per channel (or 24 bits per pixel) representation that Alan is

    Even in the case of a non-linear file format such as JPEG, the
    software (Photoshop or whatever) has already decoded it back into a
    linear representative workspace. Inside Photoshop, fully saturated
    green is R=0,G=255,B=0 regardless of the compression format used to
    store the file onto the disk.

    If you want Alan to change his measurement units based on the storage
    compression, his results or statements would be entirely meaningless
    to anyone wanting to compare the two (which was the point).
    He didn't use a binary editor to look at the RAW file itself, he's
    quoting Photoshop numbers.
    Yes, 118/255 is no where near black, even if you store the file in
    Native American Smoke Signal format. Storage makes no difference to
    what 118/255 means inside Photoshop once the file has been decoded.
    Owamanga, Mar 28, 2005
  19. 118/255 within Photoshop has no meaning without a coding.
    When you do that coding the RAW data is gone. There might
    be zillions of unlinearities before arriving att this number.

    I can give you an example. Some time ago there was som discussions
    in this forum ragarding native JPEG in the D10. It was shown that
    you lost high lights if you let the camera do its own conversion
    to JPEG. It was therefore shown that you shall NEVER let D10 do
    the JPEG compression - always use RAW.

    I think this was shown withot a doubt.

    As I have written in another reply - there are only three ISO values
    that are meaningful to compute for the camera:

    1. The ISO the light meter in the camera assumes.
    2. Saturation based ISO.
    3. Noise based ISO.

    All three can be found in this paper:

    None of those three meassurements are based upon measuring
    a grey tone in the resulting picture. A grey tone in the
    resulting picture depends on the linearity of the system.
    Therefore - you don't use a grey tone when defining
    the ISO of the system.

    OK - a solid state sensor is very linear - but not 100%.
    And there is nothing that says that the A/D conversion
    is linear. And there is nothing that says that the
    RAW import must be linear. An what about the numbers
    shown in Photoshop. What do 118/255 mean for a 16 bit
    image? For an 8 bit image? Don't forget that Photoshop
    do color management. Are the number before or after
    converting to the view color space?

    So - in short. I don't think you can use a grey tone to
    determine the ISO sensitivity for a complex system.

    Roland Karlsson, Mar 28, 2005
  20. Alan Browne

    Alan Browne Guest

    Makes sense. But where is 18% grey?
    This is all very unclear to me. Not your post. I just haven't taken
    the time to look at the math.
    Alan Browne, Mar 28, 2005
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