Film Base Color Compensation

Discussion in 'Photoshop' started by Anton Shepelev, Jul 2, 2010.

  1. Hello all,

    I got a question connected with the digital postpro-
    cessing of color film scans.

    Popular scanning software like VueScan offers what
    is called "film base color correction", which, for
    color films, is used to compensate for the inherent
    non-white color of the transparent film.

    Here I'd like to ask: What exactly does one need to
    do to compensate for film base color? Is it enough
    to shift the white point to transparent area of the
    film or not? Or, maybe, it is some non-uniform color
    transformation depending on brightness?

    I am asking this because simply making a transparent
    (not exposed) sections of the film grey (almost
    white in fact, to get the shadows on the positive
    almost black) does not seem to yield the right col-
    ors on the whole scan...

    Thanks in advance,
    Anton
     
    Anton Shepelev, Jul 2, 2010
    #1
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  2. Mike Russel:
    Thank you for the reply, Mike.

    Light/dark issues can be dealt with using curves, I
    know that, but colour is the problem for me.
    I suppose this answers my question. I was setting
    the colour (but not the brightness) of the white
    point to the colour of the trasparent section of the
    negative and hoped that all the other colours would
    become correct and the only remaining thing to do
    would be to adjust brightness levels using bright-
    ness/contrast or curves (general, not per-channel).
    But quite often I didn't like the results and manu-
    ally adjusted colours (temperature/hue) in a way
    that the transparent negative no longer was grey.

    So I thought that maybe some other kind of colour
    substraction was needed. I don't know the correct
    term for it, but I was thinking about a non-linear
    functional dependence of the white point on the
    brightness, so that not only black and white points
    but all the grey points were needed to define it.

    This idea came to me when I found that the scans of
    my very old slides exhibit a corellation between a-
    channel and brightness which I don't know how to fix
    using the standard tools...

    Anton
     
    Anton Shepelev, Jul 3, 2010
    #2
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  3. Mike Russel:
    Thank you, Mike.

    I was using Lightroom which is much more limited and
    with more complicated tools (Exposure and Blacks)
    instead of Levels, and it's Auto-Tone function,
    although reported to do the same as Auto-Levels in
    Photoshow, in fact often yields a lot of clipping.

    Now I will try Photoshop or another program with
    decent Curves.

    PS: Also skip below to my reply to sir F. A. Rien
    for some examples that I couldn't fix in Light-
    room.

    John Stafford:
    Thanks for pointing.

    Sir F. A. Rien:
    You mean manually finding a curve for each color
    channel? I shall try it. BTW, here are some exam-
    ples:

    1. http://img15.imageshack.us/img15/4029/93569140.jpg

    2. http://img695.imageshack.us/img695/7707/64327661.jpg

    3. http://img16.imageshack.us/img16/977/48563022.jpg

    Anton
     
    Anton Shepelev, Jul 4, 2010
    #3
  4. Mike Russel:
    Yes. Thank you for the advince, Mike.

    Sir F. A. Rien:
    How did you see they were clipped? The histogram is
    not trimmed from either side...
    Yes, but I scanned an already cut slide in a special
    holder. I didn't have a full roll.
    I usually set up my scanner exposure (total and per-
    channel) so that each channel is "spread" over most
    of the dynamic range, becuase my scanner is only 12
    bit per channel and this is more important than for
    16-bit.

    Also it is Ed Hammrick's (author of VueScan) advice
    to the set exposure by a transparent piece film
    thereby making it as bright as possible (but not
    overexposed). I think this is done to minimize the
    noise-to-signal ratio.

    I wonder why you say "compress" about the blacks
    (negative's whites). If they are not overexposed and
    there is no "clipping", isn't it enough? Do sensors
    compress high-signal data?
    Great thanks for doing them! I had already down-
    scaled and downsampled the images before posting
    anyway, but now I will have where to look back while
    tinkering with curves.

    Anton
     
    Anton Shepelev, Jul 6, 2010
    #4
  5. Sir F. A. Rien:
    Such is the graphical histogram, but you will not
    find any pixel with a value of 255 in any channel.
    I have a Nikon Coolscan IV film scanner (and am
    dreaming of Coolscan 5000) which has two adapters:

    · An adapter for un-mounted 6-frame film strips.
    It work most of the time, but if the film is
    badly curled (either laterally or longitudi-
    ally) it does not allow for sharp scans because
    it can't hold such film sufficiently flat. And
    both ends of the strip are especially difficult
    to get flat (and therefore sharp).

    For important frames, I usually forcus at sev-
    eral points on the film and then manually set
    the focus to a value in-between the maximum and
    minimum of the measures.

    Even for the 2900 dpi resolution the effects of
    little focus deviation (present even on seem-
    ingly flat film) are apparent. The scanner's
    lens seems to have a narrow depth of field.

    · An adapter for mounted 6-frame film strips. The
    mounting itself is a "frame" that helps to hold
    film flat by clamping the strip along the edges
    and along the inter-frame spaces. Being so fee-
    ble as to yield somewhat to strongly curled
    film's desire to spring back into a roll, it is
    nevertheless better for scanning deformed film.
    And there's lots of it in my father's ar-
    chieves.

    It also can accept single frames in the "stan-
    dard" mount, like the slides here demostrated.

    I don't know if it is possible to use another
    adapter with this scanner. A glass film holder would
    be interesting to try for it would hold any film
    perfectly flat...

    Sorry, I didn't understand what Weiss is in your
    context.
    It is a nice idea to use perforation holes for that.
    More so it is in slides where one is never sure what
    the minimal density is, because it appears on maxi-
    mally exposed areas, while in negatives you always
    have transparent film as reference point. Thank you
    for the advice.
    I hadn't done it before I ran into a film so bad
    that the range of one channel was two or more times
    narrower that the that of the others, like the red
    channel on the slides. Manually setting the indivi-
    dual exposure in the way I mentioned helps to use
    the sensor's dynamic range more effectively and to
    get a better (with more correct levels and the white
    point, and with more data) raw scan requiring less
    tweaking in the postprocessing application.
    Yes, but I was mentioning the noise-to-signal ratio
    :)
    Hmmm. I wouldn't call this compression. I'd call it
    the clipping of blacks, or underexposure. If the
    exposure is too low, densities from 0 to 16 are
    "mapped" to, say, -20 to -4 on the censor that clips
    them all to zero. And with the clipping of whites
    (overexposure) the situation is symmetric.

    Compression is a non-linear operation, like a curve
    to increase the contrast compresses blacks and
    whites, while decompressing the mid-tones. The Lev-
    els tool )(de)compresses the whole density range
    uniformly. But neither happens with change of expo-
    sure. That's how I see it.
    I didn't take this photos. They are older than I am
    and originate from my friend's collection. Also it
    is not the original film (from the camera) but a
    copy of it (which is probably the reason of the low
    sharpness). In the Soviet times, such packs were
    produced massively by Diafilm and sold at museums
    e.t.c.

    They also made "illustraded tales" based on anything
    from classics like "The Lost World" to Russian folk-
    lore. I have some of them too and am going to scan
    them. They were in the half-frame format (24 x 18
    mm), each frame consisting of a picture and several
    lines of text.

    Anton
     
    Anton Shepelev, Jul 6, 2010
    #5
  6. Anton:
    Sorry for this. In the bus on my way home from work
    it occured to me that I was wrong. Of course, com-
    pression takes place too, because a higher exposure
    setting causes the same density range on film to
    occupy a wider brightness range in the digital scan.

    Anton
     
    Anton Shepelev, Jul 6, 2010
    #6
  7. Sir F. A. Rien:
    Thank you very much, kind sir!

    http://img411.imageshack.us/img411/6350/11exp.jpg

    So old man Kokovanya, orphan Daryonka and cat Mury-
    onka began to live together. They lived without
    haste and were not avaricious and din't complain
    about their life for everybody had his own work.

    Anton
     
    Anton Shepelev, Jul 11, 2010
    #7
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