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Thread: Can someone with a transmission wedge, Vuescan and a flatbed help me?

  1. #1

    Can someone with a transmission wedge, Vuescan and a flatbed help me?

    I was using VueScan the other day and wondered about something that I hope some forum members may help me with. VueScan has a very useful exposure lock feature under the input tab which lets one lock the exposure on the various channels of scanners that support them. Now, doing this means that we can control the exposure from frame to frame and not have the scanner tweak its autoexposure.

    So I got to thinking - If I scan a calibrated transmission step wedge - the ones from Stouffer or Kodak - at a locked exposure, and then scan a negative at the same exposure on a flatbed like the Epsons (3200 to v750) comparing the greyscale values in photoshop could give us the density quite accurately.

    So, for example, you would lock the exposure at 5 and scan the transmission wedge and then a negative at that same exposure. Now, open both in photoshop in 8bit grayscale (I'm only thinking BW at the moment) and with the dropper tool read the value at a point in the picture where you want to read the density. Now in the scan of the step wedge, find the step which has the closest greyscale value to that slot. Now, of course, since it's a calibrated wedge, you know the actual density of that step and that should also be the density of the point you read in the negative (or pretty darn close).

    So, that's the theory. But I am away from home right now and don't have access to the gear to try this out. So if any forum members have a calibrated transmission wedge, Vuescan and a flatbed scanner (preferably one of the Epsons), I'd be grateful if they could try out this little experiment to see if it works.

    PS: Please don't ask me to just buy a real densitometer. I already have one. It's just that this seemed like an interesting idea and a valid way to measure densities by comparing to a known density. Thanks for any help on this. Below is the VueScan tab.

  2. #2

    Join Date
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    Re: Can someone with a transmission wedge, Vuescan and a flatbed help me?

    Quote Originally Posted by Anupam View Post
    I was using VueScan the other day and wondered about something that I hope some forum members may help me with. VueScan has a very useful exposure lock feature under the input tab which lets one lock the exposure on the various channels of scanners that support them. Now, doing this means that we can control the exposure from frame to frame and not have the scanner tweak its autoexposure.

    So I got to thinking - If I scan a calibrated transmission step wedge - the ones from Stouffer or Kodak - at a locked exposure, and then scan a negative at the same exposure on a flatbed like the Epsons (3200 to v750) comparing the greyscale values in photoshop could give us the density quite accurately.

    So, for example, you would lock the exposure at 5 and scan the transmission wedge and then a negative at that same exposure. Now, open both in photoshop in 8bit grayscale (I'm only thinking BW at the moment) and with the dropper tool read the value at a point in the picture where you want to read the density. Now in the scan of the step wedge, find the step which has the closest greyscale value to that slot. Now, of course, since it's a calibrated wedge, you know the actual density of that step and that should also be the density of the point you read in the negative (or pretty darn close).

    So, that's the theory. But I am away from home right now and don't have access to the gear to try this out. So if any forum members have a calibrated transmission wedge, Vuescan and a flatbed scanner (preferably one of the Epsons), I'd be grateful if they could try out this little experiment to see if it works.

    PS: Please don't ask me to just buy a real densitometer. I already have one. It's just that this seemed like an interesting idea and a valid way to measure densities by comparing to a known density. Thanks for any help on this. Below is the VueScan tab.
    Vuescan will function as a densitometer (within limits). Press the control key and the log density values will be displayed where the RGB values are normally shown.

    Don Bryant

  3. #3

    Re: Can someone with a transmission wedge, Vuescan and a flatbed help me?

    Quote Originally Posted by D. Bryant View Post
    Vuescan will function as a densitometer (within limits). Press the control key and the log density values will be displayed where the RGB values are normally shown.

    Don Bryant
    Thanks, but I am not looking to cobble together a makeshift densitometer. As I wrote, I have a real densitometer. I was more interested in whether the principle on which I am basing this is workable.

    I'll do the test myself once I get home in a few weeks.

    -A

  4. #4
    jvuokko's Avatar
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    Re: Can someone with a transmission wedge, Vuescan and a flatbed help me?

    I have been using vuescan as an densitometer with my epson 3200 flatbed scanner for years. Before that, I used spotmeter as densitometer.

    My approach was to measure densities that vuescan gives from stouffer wedge, then create correction funtion which maps vuescan's (or scanners) densities to real densities that wedge represents.

    Here is my correction curve as for example. The Y-axis is vuescan measured density and X-axis is real density on the stouffer wedge.

    This works fine... BUT I have to warn that scanner, like real densitometer, does not give exact readings with all materials. The color of the scanner (or densitometer) light have it's own co-effect.

    I found this recently when I measured my negatives with densitometer. The readings were bit off from my scanner-densitometer readings. The densier the negative, the more difference there was.

    However densitometer readed stouffer wedge accurately. As did my scanner when applying correction function. So for stouffer wedge, both were "calibrated". But not for all kind of silver based films.

    Which one gives more accurate reading from film X developed in developer Y remains mystery. It is densitometer or it is scanner?

    The spectrum of the light source is not only thing that causes differencies. Also the type of the light source alters the result. The light source of scanner is usually quite soft, like diffuser head. The light source of densitometer might be diffused or it might not be. Light source effect is similar like in enlarger. And that affects the measured density.

    The difference between my scanner approach and densitometer has createst impact on zone system calibration. The de-facto highlight zone for calibration is VIII, and around it's densities there are almost 0.1D difference with my scanner and densitometer I have used.

    Difference with other densitometer? I don't know, but I assume that two different densitometers would also give different readings.

    So currently my conclusion is that I will still keep using the scanner approach, but I am more aware about the fact that with certain materials (such as silver based film), the error marginal is approx. 0.1D when reading densities above 1.10


    If someone has access to densitometers from different manufactors, I would be pleased to hear if all these densitometers will show same density with same test negative, or does every densitometer show different density readings.

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