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Thread: How to build a scanning curve?

  1. #1

    How to build a scanning curve?

    In the excellent January/February 2006, View Camera Magazine article, A Simple Way to Test For Film Speed and Developing Time, Steve Simmons refers to a final step of building a scanning curve. This curve is then used to bring in your image as it proofed for you. Can anyone direct me to a web site or book which will instruct me on this? I think I understand what a film curve is, but am new to scanning and digital printing. Thanks

  2. #2
    Travelin' on the Mobius strip Chris_Brown's Avatar
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    Re: How to build a scanning curve?

    Quote Originally Posted by Stephen Sample View Post
    In the excellent January/February 2006, View Camera Magazine article, A Simple Way to Test For Film Speed and Developing Time, Steve Simmons refers to a final step of building a scanning curve. This curve is then used to bring in your image as it proofed for you. Can anyone direct me to a web site or book which will instruct me on this? I think I understand what a film curve is, but am new to scanning and digital printing. Thanks
    I can't direct you to a web site with instructions, but I can help get you started.

    The idea is to develop a set of input parameters in your scanning software that gets you acceptable results without a lot of fussing and guessing with that software.

    To do this, set up a controlled scene with:
    • A MacBeth Color Chart, for the gray scale and color swatches
    • A clean, polished chrome object, for Dmin
    • A large area of black velvet,for Dmax
    • A person, for flesh tones

    Shoot a bracket of this scene and process in your normal fashion. Make sure you color balance your film for your lights. Mount the film on your scanner and
    • Use the black velvet to set the shadow endpoint
    • Use the polished chrome to set the highlight endpoint
    • Use the gray scale to set the RGB curves for neutral gray throughout the density range
    • Use the color swatches to verify your RGB values (MacBeth color charts have very useful RGB info on the back of the chart)
    • Use the person to verify correct RGB skin tones and to visually evaluate the scan

    After you've made a few test scans and have dialed in your settings, save those settings for that film type. Then, when it comes time to scan a batch of that film, you've got an excellent starting point.

    I should point out that Don Hutcheson, a revered scanning expert, is a proponent of scanning everything in 16-bit color using only one setting that accomodates your scanners total input gamut. His arguments for this are pretty good and worth a read.

    I hope this helps.

  3. #3

    Re: How to build a scanning curve?

    Thanks Chris,

    I plan to try your suggestions soon. Also, the Hutcheson web site had some wonderful scanning resources and articles. I appreciate your input. Steve

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    Re: How to build a scanning curve?

    Quote Originally Posted by Chris_Brown View Post
    I can't direct you to a web site with instructions, but I can help get you started.

    The idea is to develop a set of input parameters in your scanning software that gets you acceptable results without a lot of fussing and guessing with that software.
    These suggestions are quite reasonable, but let me note that they assume you are using transparency film. For negative film, color or b/w, the second and third should be reversed.

    To do this, set up a controlled scene with:
    • A MacBeth Color Chart, for the gray scale and color swatches
    • A clean, polished chrome object, for Dmin
    • A large area of black velvet,for Dmax
    • A person, for flesh tones

    Shoot a bracket of this scene and process in your normal fashion. Make sure you color balance your film for your lights. Mount the film on your scanner and
    • Use the black velvet to set the shadow endpoint
    • Use the polished chrome to set the highlight endpoint
    • Use the gray scale to set the RGB curves for neutral gray throughout the density range
    • Use the color swatches to verify your RGB values (MacBeth color charts have very useful RGB info on the back of the chart)
    • Use the person to verify correct RGB skin tones and to visually evaluate the scan

    After you've made a few test scans and have dialed in your settings, save those settings for that film type. Then, when it comes time to scan a batch of that film, you've got an excellent starting point.
    Let me also add that a typical gray scale won't usually include a sufficient range of values. Instead, I would recommend varying lighting over the scene and spot metering various uniformly lit areas in the scene. For negative film, you should aim for as many EV values as possible over a an 8 to 10 stop (or higher) range. For transparency film, a narrower range of EV values might be acceptable.

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    Re: How to build a scanning curve?

    Let me raise another point, which I've been trying to emphasize, without apprent success.

    When you scan, the software typically will allow you to read RGB values in the range 0 to 255. By modifying various scanning paremeters, you can change these values, and they are what you need to set correctly so the result will look the way you want it to, either on your monitor or in a print. (Printing also will require calibration.) Unfortunately, the scanning software often seems to have a will of its own in setting these RGB values. Very similar scenes may yield different such values even when you think everything has been standardized. For this reason, it is helpful to be able to read the raw densities above base. This is closer to what the scanner reads from the film and which the scanning software uses to determine the RGB values by a somewhat complicated algorithm. Some scanning software, such as Vuescan, allows you to read the densities above base. I find it easier to get a consistent idea of what is on the film using these.

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    Ted Harris's Avatar
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    Re: How to build a scanning curve?

    Stephen, the suggetions so far are good. Especially don't ignore doing all your scanning in RGB and that this method is a starting point. Once you set up a workflow of this sort you still need to evaluate each individual negative/transparency and make the best setting sfor that particular piece of film, just as you would do if you were enlarging the film. For many properly exposed peices of film this will simply mean setting the white and black point for that particular image and you will often find that your software (especially if you are using Silverfast Ai) has already chosen the best points but you do need to do this for every scan if you want optimum results.

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    Re: How to build a scanning curve?

    Quote Originally Posted by Stephen Sample View Post
    In the excellent January/February 2006, View Camera Magazine article, A Simple Way to Test For Film Speed and Developing Time, Steve Simmons refers to a final step of building a scanning curve. This curve is then used to bring in your image as it proofed for you. Can anyone direct me to a web site or book which will instruct me on this? I think I understand what a film curve is, but am new to scanning and digital printing. Thanks
    I understand doing this for scanning transparencies. For negatives, either color or B&W, it doesn't make much sense to me.

    The reason is that a properly exposed tranny usually has a density range that is both large and relatively consistent. It has to if it is to perform its primary task which is image projection. It has to have a black that is black and a white that is white. Not relatively black or white, but actually black or white. Since the density range is relatively predictable, it makes some sense that one could find a scanner setup that will work relatively well with most every tranny.

    Not so with negatives. The density range of negatives varies widely. It's not possible to find a single scanner setup that will work relatively well with most every negative, particularly B&W negatives.

    The best workflow I've been able to come up with for drum scanning my negatives is to set the black and white points for each channel (R, G, or B) individually, setting the log amp circuits to give their full range of responses to these limited density ranges. This results in a scan file that is very close to the proper contrast for each color, and minimizes color correction work in photoshop for me. This works particularly well for my B&W scans.

    Profiled scanners typically result in negative scans that are both flat and sparse. By sparse I mean that the histograms show that the scan data doesn't fill the digital range well. This means one has to chop off the empty ends of the histograms using the photoshop levels tool for example. This in turn raises the risks of posterization as photoshop has to spread this sparse data over a larger digital range.

    I'm afraid I'm not being very articulate this morning. Sigh... What I'm saying is, if you are scanning trannies go ahead and make a profile for the scanner if you want. If you are scanning negatives, I think you'll be wasting your time. But you can try it and come to your own conclusions.

    Bruce Watson

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    C. D. Keth's Avatar
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    Re: How to build a scanning curve?

    I beg to differ, Bruce. Unless one is doing something seriously wrong, most of one person's negatives of a given filmstock, processed the same way, and exposed the same way will tend to have very close d-min, d-max and gamma. Making a scanning curve for this situation will get you 95% to what you need, with the last 5% being for variation in subject, mostly. I think it's worth doing for the time it will save you making the same corrections over and over.

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    Travelin' on the Mobius strip Chris_Brown's Avatar
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    Re: How to build a scanning curve?

    Let me also add that a typical gray scale won't usually include a sufficient range of values. Instead, I would recommend varying lighting over the scene and spot metering various uniformly lit areas in the scene. For negative film, you should aim for as many EV values as possible over a an 8 to 10 stop (or higher) range.
    This assumes that the material being lit (e.g., white seamless) has no color bias and the lights used have been balanced to the film (i.e., targeted to a specific color temperature and green/magenta balance).

    I prefer the MacBeth chart because the swatches are neutral and without color bias. If more than five data points are needed on a gray curve, I think it'd be a waste of time - adjustments will have to be made for the actual photos to be scanned (not the test image).

    For negatives, either color or B&W, it doesn't make much sense to me. (...) The density range of negatives varies widely. It's not possible to find a single scanner setup that will work relatively well with most every negative, particularly B&W negatives.
    So, you're saying you begin your neg scanning sessions with no presets in the scanning software? It seems you're re-inventing the wheel each time.

    If the neg density changes in negs due to intentional variations in processing, then a test neg could be made for each push or pull scenario. That's a lot of work for the extra mileage, unless you push/pull frequently.

  10. #10
    C. D. Keth's Avatar
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    Re: How to build a scanning curve?

    Quote Originally Posted by Chris_Brown View Post
    If the neg density changes in negs due to intentional variations in processing, then a test neg could be made for each push or pull scenario. That's a lot of work for the extra mileage, unless you push/pull frequently.
    Exactly what I think. If you have THAT much variation in your processed negatives, there are more serious problems than your scanner curves.

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