why separate targets are provided for different film stocks (Fuji Velvia/Provia and Kodak Ektachrome) if the goal is to calibrate the scanner, not the film?
Yes, I am interested in transparent targets only.The paper-based ones can be made fairly simply by just outputting a digital file to any reasonable quality color printing service that outputs on chromogenic paper.
The gamut of each film is different and ultimately, the profile won't be just scanner-specific, it will be specific to the combination of scanner + film.
So, if we shoot the same scene with two color reversal (slide) films and use the corresponding calibration sets the digital images should be identical (under ideal conditions of identical exposures and processing, of course).
Why is the difference preserved throughout calibration?no, that's not correct as far as I understand it. A calibrated Velvia scan and a calibrated Kodachrome scan from the same scene will come out looking differently in the digital domain just as the actual slides look differently, but the digital on-screen version should look virtually identical to the original slide in both cases. That's what you calibrate for.
The main question is how much utility there is in calibrating the scanner with targets on different types of film, since you'd theoretically expect that calibrating against a single type of slide should yield good results for other types of slide film as well. However, in practice, there will likely be minor differences that result from the unique ways in which different emulsions absorb and transmit light, for instance, which interacts in turn with the spectral sensitivity of the scanner.
By a "Velvia target," do you mean that an IT8 calibration target was photographed on Velvia?
is it just an IT8 target printed on a slide that has been bleached of all silver, dye, etc;
Yes, I am interested in transparent targets only.
So, if we shoot the same scene with two color reversal (slide) films and use the corresponding calibration sets the digital images should be identical (under ideal conditions of identical exposures and processing, of course). Is there a way to retain the film character and calibrate just the scanner part? If I am scanning some old Kodachromes I would definitely like the digital image to keep the unique look of this film. The same is true for Fuji Velvia.
Yes, I am interested in transparent targets only.
So, if we shoot the same scene with two color reversal (slide) films and use the corresponding calibration sets the digital images should be identical (under ideal conditions of identical exposures and processing, of course). Is there a way to retain the film character and calibrate just the scanner part? If I am scanning some old Kodachromes I would definitely like the digital image to keep the unique look of this film. The same is true for Fuji Velvia.
A calibrated Velvia scan and a calibrated Kodachrome scan from the same scene will come out looking differently in the digital domain just as the actual slides look differently, but the digital on-screen version should look virtually identical to the original slide in both cases. That's what you calibrate for.
Yes, my interest is mostly academic. I have no need to produce colorimetrically correct scans of Kodachrome slides.so this may be an academic exercise
And so is mine. However, sometimes I scan historical slides made by other photographers and I would like my output to be at least consistent, if not accurate.My thinking on this is it's my photograph and my creation.
I am struggling to understand why multiple film stocks. Shouldn't one be sufficient to calibrate the scanner provided the XYZ / Lab values of the target patches are known? Does it even need to be a photographic process?
I believe we can exclude the color perception of human eye from the equation. It comes into play at a later stage when viewing the digital image on a computer screen, or a print on transparent media on a light table. Both presentations introduce a whole lot of their own transformations to the image so let's focus just on the transparency to digital image transform.In reality, a magenta patch on two different pieces of slide film that appear more or less the same color to the human eye may in fact have subtly different transmission densities.
Moreover, the scanner's sensor sites (typically CCD) have their own spectral sensitivity curves.
Till this point, it's all good. To rephrase: (1) we have spectral sensitivity curves for the three color channels of the scanner which most likely differ from that of the dyes in the transparency; (2) the scanner applies some built-in corrections to compensate for this discrepancy (color space transformation?); (3) this compensation is not ideal and can be improved by using a IT8.3 calibration target. It has enough color patches to build a fairly detailed 3D Look-up Table (LUT) that can be applied to a scanned transparency.Part of this is filtered out by the scanner's software/firmware on the basis of calibration procedures that are done during product development (which would boil down to pretty much what you were saying).
I believe we can exclude the color perception of human eye from the equation.
Yes, my interest is mostly academic. I have no need to produce colorimetrically correct scans of Kodachrome slides.
And so is mine. However, sometimes I scan historical slides made by other photographers and I would like my output to be at least consistent, if not accurate.
I believe we can exclude the color perception of human eye from the equation.
If I understood you correctly, you are saying that if the same film stock is used to make the calibration target and the transparency, the calibration uncertainties will be smaller and the scanned digital image will be more accurate.
This is called scanner metamerism problem.
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