Using filters with Tri-X

Discussion in 'Digital Cameras' started by Peabody, Sep 11, 2013.

  1. Peabody

    Peabody Guest

    I know it says rec.photo.digital, but the experts here will know.

    I have an old 6x9 medium format camera, along with a Wratten 23A filter,
    which is kinda light red, and I'm going to shoot a roll of Tri-X. In
    researching that filter on line, I find a variety of answers for how many
    stops it costs - anywhere from 2 to 3 stops, which doesn't exactly pin things
    down.

    I understand that the exposure adjustment needed depends on what's in the
    scene, but I wonder if it would make sense to get out my digital camera, and
    see what adjustment I need to make when using the filter and shooting a solid
    white subject, versus shooting the same subject without the filter. It seems
    that should give a pretty good value for the number of stops this filter uses
    - on average. But of course that would be the case only if the response of
    the digital camera is similar to that of Tri-X film.

    At a cost of over $2 per frame just to get negatives, I just don't want to
    waste money on bracketing. So I'm just looking for a good ball-park
    adjustment that I can use when using this filter.
     
    Peabody, Sep 11, 2013
    #1
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  2. Peabody

    Savageduck Guest

    Why would you need that particular filter for shooting the scene you
    have in mind?
    Will you need to darken skies, deepen shadows and increase contrast?
    Perhaps using your digital camera to take a "digital proof" of your
    target scene would give you more useful exposure information than
    shooting a white target. Though getting a close match on lens
    characteristic, and ISO sensitivity is probably going to be tricky.

    Then some digital cameras have flexible B&W modes. My D300S for
    example, in B&W mode provides for the use of in-camera B&W color
    filters, which can give you a pretty good idea of how each filter can
    effect the captured scene. However shooting your digital camera in B&W
    mode with the filter in place could also give some useful exposure data.

    With NIK Silver Efex Pro2, those same filters can be applied during the
    B&W conversion process.
    ....and that ball-park adjustment, right or wrong is going to be a guess.

    I have a feeling that if you are not familiar with the performance of
    your camera with Tri-X without this filter, you will probably need to
    invest in some sacrificial bracket shots, with at least one no-filter
    control shot included in your experimental shots.
     
    Savageduck, Sep 11, 2013
    #2
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  3. Your theory is sound enough, but the complexity of
    calibration is probably excessive.

    Ideally you could take a pair of shots with and without
    the filter of a good test image and then shoot the same
    image with a digital camera to provide all of the data
    needed to work out the best match of the digital
    camera's JPEG engine parameters.

    Taken to extremes, using a commercially produced test
    image and a densitometer, it is probably possible to
    generate a tone curve that can be loaded into the
    camera, which along with the camera's standard
    desaturation adjustments, can produce a BW JPEG that is
    virtually identical to what the film will produce.

    On a simpler level it might be possible to just use
    empirical experiments to find settings that are close
    enough. Basically you'd adjust Contrast, Hue and
    WhiteBalance of the JPEG engine to get as close as you
    can to the same tone mapping produced by the film.

    Even if going with the simple method it might be wise
    to purchase or even print a copy of an IT8 Color Chart.
     
    Floyd L. Davidson, Sep 11, 2013
    #3
  4. Peabody

    Peter Irwin Guest

    My old Kodak sheets say filter factor 5 (2 1/3 stops) for daylight.

    For most purposes it is better to err slightly on the side
    of generous exposure. If you are after dramatic skies you
    might want a slight underexposure.
    Generally speaking, metering through a filter results in a slight
    underexposure because meters (and digital cameras) do not have
    the oversensitivity to blue that black and white films do.

    Red filters, even fairly light ones like your 23A, will make the
    colour rendering of your pictures somewhat wonky. You can get some
    idea what it will do by looking through the filter, but your digital
    camera should come in handy for this if you use the filter in front of
    it and view in greyscale. The 23A is not a general purpose,
    looks-good-on-everything filter like the 8, 9, and 11 filters.
    Developing black and white film is something that most people can
    do a better job of for themselves than they can get someone else to
    do for money. It costs about $50 to get started, but you should be
    able to it for less than a dollar an exposure even with film at
    $5 a roll of eight exposures.

    Peter.
     
    Peter Irwin, Sep 11, 2013
    #4
  5. Peabody

    Alan Browne Guest

    If shooting Tri-X I don't think you should be too frightened to shoot
    with 2.5 or 3 stops compensation for the filter. The film is tolerant
    to a high exposure.

    Using a digital camera to measure the difference may help. So would a
    spot meter.

    Digital sensors are more akin to slide film than negative - but that
    shouldn't matter very much where measuring a difference due to the
    filter is concerned.

    I'll give it a try tonight with my meter.

    --
    "Political correctness is a doctrine, fostered by a delusional,
    illogical minority, and rapidly promoted by mainstream media,
    which holds forth the proposition that it is entirely possible
    to pick up a piece of shit by the clean end."
    -Unknown
     
    Alan Browne, Sep 11, 2013
    #5
  6. Peabody

    Peabody Guest

    Thanks very much for the responses.

    I have two filters with the camera, the 23A light red that I mentioned
    before, and a K2 yellow. The sources I've found are pretty consistent about
    the K2 being a good filter for almost everything outdoors - not to produce
    dramatic shots, but just to correct for the film's over-sensitivity to blue.
    So in effect, the yellow filter just gets you back to natural looking skies
    in B&W, instead of white skies. And everything I've found so far says to add
    one stop of exposure when using the K2.

    I understand the 23A produces a much more dramatic effect, with dramatically
    darkened skies and therefore comparatively brighter clouds. But the exposure
    adjustment varies with the source: 2, 2 1/3, or even 3 stops in one case. So
    I was just trying to find a way to calibrate that filter using my digital
    camera shooting a white subject, although it may be that a green subject
    would work as well as an "average" color. I can't really use blue because
    the whole point is to expose the subject properly but deliberatey darken the
    sky - although the effect of the filter on a blue sky would certainly provide
    the maximum number of stops lost with the filter. And I assume the least
    loss would occur with a red subject, with green being in between.

    So I will play with the filters on my Canon A590 P&S, and my T2i, and see
    what results I get. I would need to put the T2i on spot metering because the
    filters aren't big enough to cover the lens.

    Well, I'll post more when I've done some testing.

    And yes, I'll be taking a digital camera along when I shoot the film camera,
    basically to use as a light meter. It's just that the filters complicate
    things a bit.
     
    Peabody, Sep 11, 2013
    #6
  7. If in doubt negative film can handle slight-overexposure. I recall a
    Wratten 23 was about 2.5 stops. But it would be safe to overexpose it a
    tad. As the camera is old I would be more concerned with the shutter
    being slower than marked. It is a good idea to exercise the shutter
    before running film in the camera. Get the lubricates moving.
     
    Usenet Account, Sep 11, 2013
    #7
  8. Peabody

    J. Clarke Guest

    Before I go into technical matters, you're really working too hard at
    this. This is Tri-X you're talking about. It's probably the most
    forgiving film ever produced. If you're in the ballpark and don't blow
    out the highlights the results will be fine. 2, 2-1/3, 2-2/3, 3, it
    doesn't really care.

    The reason you're getting different stories on the exposure adjustment
    is that it is different for different films. For example, Kodak's
    datasheet says to use a filter factor of 10 for the 25 with Tri-X motion
    picture reveral film or 8 for the 25 with Tri-X negative (both called
    "Tri-X but different films). Unfortunately I can't find a datasheet
    that shows the 23A with Tri-X negative.

    The datasheet for Tri-X motion picture reversal says to use 5 for the
    23A. On that basis my gut reaction is to try 4 for regular Tri-X.

    The filter factor translates to f-stops using the formula "f-stops = ln
    (filter-factor)/ln(2). Plugging in the numbers that would mean an
    exposure compensation of 2 stops even.

    The datasheets I found are at:

    <http://www.kodak.com/global/en/professional/support/techPubs/f4017/f401
    7.pdf>
    and
    <http://motion.kodak.com/motion/uploadedFiles/7278_TI0603.pdf>

    Note also that the datasheets assume that metering is using light
    reflected from an 18 percent gray card at the location of the subject.
     
    J. Clarke, Sep 11, 2013
    #8
  9. Peabody

    George Kerby Guest

    Most decent filters print the exposure factor on the rim, along with
    diameter, name, etc.

    If that won't, do like we did in the old days: Shoot a Polaroid test ;-)

    Good luck and have fun...
     
    George Kerby, Sep 11, 2013
    #9
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