DX-code exposure latitude code clarification requested.

Discussion in '35mm Cameras' started by Alan Browne, Feb 12, 2004.

  1. Alan Browne

    Alan Browne Guest

    http://www.geocities.com/thombell/dx.html#latitude

    Shows the 'code' for the exposure latitude portion of the 135 format DX
    code.

    On a roll of Velvia, the most pesimistic case is selected by the code,
    eg, per the table in the link above +/- 1/2.

    Now the latitude of Velvia is at least two stops either way, possibly
    +/-2.5 ...

    So does the DX code for exposure latitude have an offset base? (eg: 1.5
    or 2 would seem to make sense).
    eg does the +/- 1/2 really mean +2 +1/2 to -2 -1/2 (+2.5 to -2.5)?

    Or is the code really meant to drive the program exposure mode s/w with
    an idea of how loose or tight the s/w should interpret the scene?

    One tidbit I found via Google suggests it merely helps the camera in
    program mode to determine if it can keep the shutter fast enough to
    avoid camera shake and still get a reasonable exposure. In this case,
    only the under exp side would be needed, so seems like a partial
    response at best.

    All replies apreciated.

    Cheers,
    Alan.
     
    Alan Browne, Feb 12, 2004
    #1
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  2. Alan Browne

    Bill Tuthill Guest

    No the dynamic range of Velvia is +2/-2 or +/-2.5 if you prefer,
    but exposure latitude is not the same thing as dynamic range.
    For most normal scenes with brightness ranges of 4 stops or more,
    you could not expose Velvia 50 at EI 12 or EI 200 and still get
    acceptable results.

    That's what exposure latitude means: acceptable results at a
    given number of stops over and under. Most print films can
    easily handle +2/-2 and some more than that.
    Yes, but I don't know of any cameras that actually make use of it.
     
    Bill Tuthill, Feb 13, 2004
    #2
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  3. Alan Browne

    Alan Browne Guest

    No matter what tolerance "exposure latitude" they put on the DX-code,
    the exposure range will not change as you change the center (18% grey)
    point... , eg: if the exp. is set for 1/10 of a stop to overexpose, then
    that much more detail will appear in the shaddows, and that much more
    detail will be lost in the highlights. +/- 1/2 latitude means nothing.

    Exp. latitude DOES mean something for negative film, as you're going to
    make prints from it and you have a lot of latitude for correction. But
    for reversal it seems like a meaningless term.
    I now think that is its only purpose, eg: useless to anyone but P mode
    users.
     
    Alan Browne, Feb 15, 2004
    #3
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