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  1. #171

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    Quote Originally Posted by Michael Hoth View Post
    thx,i used a step wedge-negative placing in the hasselblad filmholder, photographing a white card placing on zone X; did this with 5 films! maybe the displacing of the negativ leads to an uneven illumination (i lost steps 30/31 and 1/2)
    should i repeat shooting 5 films
    I downloaded your photo and looked at pixels in the two #16 wedges in Gimp. Densities by the centre are the same, but they're different by the outside. No need to repeat development. Just re-measure the wedges at the inside.
    By eliminating the jump, you can compute CI accurately.

    HTH,

    Mark Overton

  2. #172
    Stephen Benskin's Avatar
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    One problem with the in camera contracted step tablet test is that lenses are brighter in the center than the edges. A nice long lens with a good size image circle would be the best way of minimizing it.

  3. #173

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    Quote Originally Posted by Stephen Benskin View Post
    One problem with the in camera contracted step tablet test is that lenses are brighter in the center than the edges. A nice long lens with a good size image circle would be the best way of minimizing it.
    Yes!
    My Stouffer chart is back-lit with doubly diffused light.
    Shooting with a 50mm macro lens onto 35mm film got corner-falloff, creating dips in the curves.
    Switching to a 135mm lens with an extension tube, stopped down 1.5 stops, solved the problem.
    FP4+ and HP5+ now have straight line curves from centre to edge back to centre of the Stouffer.

    Also, I shoot the upper and lower halves of the Stouffer chart in separate exposures, so that each shot only has a 5-stop range instead of 10-stops. This almost eliminates flare that would otherwise push up the toe-area.

    The result of all this is almost as good as contacting.

    Mark Overton

  4. #174

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    Mark, how are you determining speed/EI photographing the backlit wedge?

  5. #175

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    Quote Originally Posted by Michael R 1974 View Post
    Mark, how are you determining speed/EI photographing the backlit wedge?
    You'll laugh at this: I guess based on where various films hit .1 over B+F!
    That's their ISO speed-point, and I know where that should be on the log E axis. Of course, developers shift that point, and there's no guarantee that the films themselves hit it exactly. But this gets me close. All my work has been comparative, usually comparing with XTOL's curves, so I haven't needed accurate log E numbers.

    Mark

  6. #176
    Stephen Benskin's Avatar
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    Quote Originally Posted by albada View Post
    You'll laugh at this: I guess based on where various films hit .1 over B+F!
    That's their ISO speed-point, and I know where that should be on the log E axis. Of course, developers shift that point, and there's no guarantee that the films themselves hit it exactly. But this gets me close. All my work has been comparative, usually comparing with XTOL's curves, so I haven't needed accurate log E numbers.
    Mark, there is a way to quantify those results. It's not perfect as you can't be certain of the metered exposure, so any variance of the film speed can't be attributed with certainty to any one factor. The basic idea is to know the value of the metered exposure. Once you know this, you can determine the exposure value at what ever additional stops is given for the test. Next, know the target exposure at the speed point for the the speed of film being tested. You can then calculate what step tablet density that should fall at for a given speed with your chosen exposure value. The attachment breaks this technique down. It's from an analysis of a method proposed by Schaefer, but it should be applicable to your situation.

    BTW, if your interested in learning more about the ISO speed point and what it means, check out the thread http://www.apug.org/forums/forum48/1...-bill-etc.html ,ISO speed determination constants - question for Stephen, Bill etc.

    Click image for larger version. 

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  7. #177

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    Perhaps this Schaefer thing is what I'm missing. I've never quite understood how to accurately find an EI when contacting in the camera. Suppose I'm targeting a fixed density of .1 above B+F, how do you relate a meter reading of a white card target to densities produced through the step wedge?

  8. #178
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    Perhaps this Schaefer thing is what I'm missing. I've never quite understood how to accurately find an EI when contacting in the camera. Suppose I'm targeting a fixed density of .1 above B+F, how do you relate a meter reading of a white card target to densities produced through the step wedge?
    Michael, it's all pretty straight forward, although Schaefer made a wrong assumption. The process is the same as with a sensitometer which is just an accurate exposure device. Opacity is incident light / transmitted light. Density is the log of Opacity. Metered camera exposure is 8 / ISO. Opacity comes from the step tablet and incident light is based on the metered camera exposure. These are the two variables needed to calculate the transmitted light for a given step tablet density. Transmitted light = Incident Light / Opacity. The film speed equation is 0.80 / Hm. What the exposure needs to be for a certain film speed is 0.80 / ISO.

    I decide which step I want to be the speed point, then determine the exposure necessary to achieve it. This becomes my standard exposure for a given speed film.

  9. #179

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    Thanks for this, Stephen. I will try this and see what happens. I may have some follow-up questions.

    Michael

  10. #180
    Bill Burk's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    Metered camera exposure is 8 / ISO. Opacity comes from the step tablet and incident light is based on the metered camera exposure. These are the two variables needed to calculate the transmitted light for a given step tablet density. Transmitted light = Incident Light / Opacity. The film speed equation is 0.80 / Hm. What the exposure needs to be for a certain film speed is 0.80 / ISO.
    So, basically 1/10th the light arithmetically from the metered camera exposure... Or 1.0 logarithmically speaking, 3 1/3 stops down from the metered point. Was it just chosen to be easy to be remembered?



 

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