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  1. #11
    Regular Rod's Avatar
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    http://en.wikipedia.org/wiki/Filter_factor

    It's all there.

    BUT... Mark has given you the best answer.

    f
    stops is a stupid way to compensate. It is duration of exposure we will mostly be interested in if we want to retain depth of field...

    RR
    Last edited by Regular Rod; 01-30-2014 at 11:26 AM. Click to view previous post history.

  2. #12

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    There are indeed plenty of tech sheets out there. But for TMax films, to keep it simple in daylight, for lt yellow or lt yellow-green, I just round the compenstation to 1 EV (either one speed slower or one stop wider open), for moderately deep green or red-orange, 2 EV,... for no.25 true red, 3 EV. I'd avoid a 29 red because it's harder to view thru and won't add much, unless you're aiming for something between red and IR, but it would also amt to 3EV with these particular films.

  3. #13
    brian steinberger's Avatar
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    Quote Originally Posted by Regular Rod View Post

    f[/I] stops is a stupid way to compensate. It is duration of exposure we will mostly be interested in if we want to retain depth of field...

    RR
    I change shutter speed stops, not aperture to retain depth of field. It's easier for me to remember how many stops of light a filter eats up then to remember what new ISO my film becomes each time I put a filter on. But to each their own. There are many ways to skin a cat!

  4. #14
    Alan Klein's Avatar
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    Interestingly, the B+W 90 light red (same as 25) I bought has a filter factor of 5 while their dark red 091M (same as 29) has a filter factor of 8. It seems their filter let through more light than the filter chart.

    Quote Originally Posted by Regular Rod View Post
    http://en.wikipedia.org/wiki/Filter_factor

    It's all there.

    BUT... Mark has given you the best answer.

    f
    stops is a stupid way to compensate. It is duration of exposure we will mostly be interested in if we want to retain depth of field...

    RR

  5. #15

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    To pin all this down precisely you have to do a series of controlled tests under specific light conditions with specific films, then measure the
    result with a densitometer. Not all pan films are by any means identical with respect to spectral response. Some have a little more sensitivity
    in the red, while others have less, and something like ACROS is technically an orthopan, with decreased red sensitivity. These kinds of differences will determine whether the factor is the same for a 25 and a 29, whether they are different, or whether a 29 just lops an entire zone from the shadows, as in ACROS, where use of one is counterproductive. Similarly, slight differences exist with respect to blue and green sensitivity, which all gets tangled up with significant color temp differences, esp apparent at high altitudes under blue skies. On and on. For critical work in the lab I measure each of these factors. In the field it will drive you nuts, and for most black and white applications, being a third stop off here and there isn't going to have much practical significance. There are also slight differences with respect to how different
    manufacturers label their filters. But to say f-stops are never useful for controlling the correction factor is misleading, particularly when recip
    factors come into play at long exposures, or subject movement mandates keeping shutter speed up, or the ability to handhold a camera in
    the case of small formats. We have choices and use them wisely, depending on specific circumstances.

  6. #16
    Alan Klein's Avatar
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    Good points all. Well I'm going to go ahead and try all three on the same subject with blue skies and clouds and bracket them as well and see what happens. Should I bracket +1 and -1 or +2 and -2?

  7. #17
    markbarendt's Avatar
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    The only reason I would bracket is if I was doing a test or if I thought my subject blinked.
    Mark Barendt, Beaverton, OR

    "We do not see things the way they are. We see things the way we are." Anaïs Nin

  8. #18
    Alan Klein's Avatar
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    Why wouldn't you bracket?

  9. #19

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    Metering thru a filter tells you how the spectral sensitivity of your meter senses a luminance chance, not how the film itself sees the difference.
    That's why Fred Picker marketed a modified Pentax Spotmeter with a special filter pack in its to correct the sensitivity to "normal" panchromatic
    film, whatever that meant. So that idea might have improved certain scenarios but not others, because not all films are the same. After awhile
    this all just starts going around in circles anyway. A little bit of practice shooting and printing with specific films, with specific filters, and
    it starts getting real instinctive. So I guess the moral of this story is, don't carry more filters than you actually need, and don't worry too
    much if your factor isn't hypothetically dead-on.

  10. #20
    markbarendt's Avatar
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    Quote Originally Posted by Alan Klein View Post
    Why wouldn't you bracket?
    Composition problems or subject problems are far more likely for me than exposure errors. I don't think I'm odd in this respect either.

    So rather than bracket, I like to spend my extra film and time alternative compositions or poses.
    Mark Barendt, Beaverton, OR

    "We do not see things the way they are. We see things the way we are." Anaïs Nin

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