Scanner Profiles and Negafix

Problems with HiRePP®, NegaFix or profiles

bosveldboer
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Scanner Profiles and Negafix

Postby bosveldboer » Mon Jan 18, 2016 11:48 pm

Hi,

After browsing the forums, I am confused about how Silverfast uses scanner profiles and Negafix film profiles.

I read somewhere that Negafix does not use scanner profiles, but uses it's own film profiles. Is this correct?

If this statement is correct, what is the use of calibrating (profiling) the scanner with IT8 targets if you are going to scan negatives?

Surely the correct process should be:

Scanner scans the negative
Silverfast applies the scanner profile to ensure that the negative image is converted to a "standard"color space by eliminating any deviations of the particular scanner
Silverfast applies the Negafix film profile
Silverfast convert the image to the internal working space set in Preferences


Is this process correct?

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LSI_Ketelhohn
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Re: Scanner Profiles and Negafix

Postby LSI_Ketelhohn » Tue Jan 26, 2016 3:48 pm

Dear customer,

That is correct, the IT8 calibration is only used for scanning positives.
It is not necessary to calibrate if you only scan negatives.
The calibration makes sure the entire colorspace of the scanner is used. For slides/positives this is important because they might actually contain information at the edge of the available colorspace.
For Negatives the information is compressed in the orange mask. The conversion uses special profiles which are adapted to the individual scanner model. An individual calibration however is not necessary.

Kind regards,
Arne Ketelhohn.

bosveldboer
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Re: Scanner Profiles and Negafix

Postby bosveldboer » Sun Jan 31, 2016 1:51 pm

Sir,

A default scanner profile is provided by the manufacturer to map the measured color values to the actual colours that the scanner is scanning.

The reason a scanner is calibrated, is to compensate for any deviation of the actual scanner response form the default response that the manufacturer assumed when the default scanner profile was created. Changes with age and use are prime reasons for calibrating a scanner to ensure that the measured response is mapped to the correct colour values.

Surely this also applies to the scanning of negatives too?

Even if the color space of a negative is compressed in the orange mask, (as stated by you), any deviation of the actual scanner response from the standard response of the scanner as stated by the manufacturer, will influence the accuracy of the measured colours and there the accuracy of the Negafix conversion?

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LSI_Ketelhohn
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Re: Scanner Profiles and Negafix

Postby LSI_Ketelhohn » Tue Feb 09, 2016 3:45 pm

Dear customer,

No that is not necessary. NegaFix autoadjusts to the compressed curves anyway.
Only positives require a calibration because they reach the edges of the available colorspace.
For Negatives these variations do not matter.

Kind regards,
Arne Ketelhohn.

stevebow
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Re: Scanner Profiles and Negafix

Postby stevebow » Wed Dec 14, 2016 1:08 am

Hi Arne,

Could you please expand on this? I have often wondered why IT8 profiling is not considered when scanning negs.

Let's say my aging scanner is loosing sensitivity in the red channel. This is very obvious visually when scanning a positive image, and an IT8 calibration will compensate for this problem.

A scanned image is a scanned image, regardless of the image source type (neg/transparency), so the "colour" of the orange mask of a given scanned negative will look different scanned when the scanner is old compared to new. If SF Ai does not use IT8 calibration when scanning negs, how does it compensate for image sensor variation/degradation over the scanner's lifetime?

Steve

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Re: Scanner Profiles and Negafix

Postby LSI_Ketelhohn » Fri Dec 16, 2016 5:13 pm

Dear Steve,

The NegaFix profiles themselves are used as input profiles in negative mode instead of the regular IT8 profile.
The compression of the image information inside the orange mask normally makes an individual calibration unnecessary. This because the calibration mainly makes sure that the outer areas of the colorspace are set correctly. For Slides this is critical but negatives normally have their data positioned well inside the available colorspace. That way even color casts created by the scanner normally can be compensated, because they do not cause image data to fall outside the available color range.

Please contact us through the SilverFast 8 Help menu's "Request online support" entry.
That way one of my colleagues or myself can give you more precise advice regarding any difficulties you might have.
We might be needing some screenshots and samples to check.

Kind regards,
Arne

Rich P
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Re: Scanner Profiles and Negafix

Postby Rich P » Sat Dec 17, 2016 9:42 am

stevebow wrote:Hi Arne,

Could you please expand on this? I have often wondered why IT8 profiling is not considered when scanning negs.

Let's say my aging scanner is loosing sensitivity in the red channel. This is very obvious visually when scanning a positive image, and an IT8 calibration will compensate for this problem.

A scanned image is a scanned image, regardless of the image source type (neg/transparency), so the "colour" of the orange mask of a given scanned negative will look different scanned when the scanner is old compared to new. If SF Ai does not use IT8 calibration when scanning negs, how does it compensate for image sensor variation/degradation over the scanner's lifetime?

Steve


I think the reason is that with a positive image the image itself is the absolute reference of how it should look and so a scanner just has to faithfully replicate what is there. The scanner is already designed for this purpose and a custom IT8 profile attempts to correct for any deviations by measuring a handful of know colour patches to modify the scanner's response. (I think this is flawed too and will present my case below!)

A negative is obviously not the final image since it comprises clouds of cyan, magenta and yellow dyes behind an orange mask. The incorrect assumption often made is that if you can faithfully replicate the cyan, magenta and yellow image (combined with the orange mask) then you can faithfully replicate its inverse. There is no industry standard method of removing the mask and inverting a negative. This was originally done optically during the wet printing process by a technician skilled in adjusting the CMY filter packs to suit. Every film stock is different and may vary further according to exposure, development and storage methods.

If you open the Advanced part of NegaFix on one of your scans and click the Expansion tab you can see the fairly narrow space occupied by the 'data' on the film. Firstly you can see how low contrast colour negatives are, which is added later according to the photographic paper used (optical printing) or in software (scanning). Secondly you will see that to remove the mask the delimiters at the start and end of each histogram are numerically all at very different places, as are the histograms themselves in relation to one another.

It doesn't really matter if any given colour channel loses some sensitivity because you (or the software) still has to go through the process of determining the start and end points of each histogram and their final positions relative to one another, which negates any issues of where the histogram starts out - there is plenty of empty space either side!

With regard to scanning slides, I have had infinitely better results turning colour management off. Initially I get a red cast in shadows, suggesting the scanner's red channel is fairly dominant but one click of Levels in Photoshop restores the correct relationship and the colours are far superior to an IT8 colour managed scan. Why? The IT8 target has a handful of known colour patches from which it constructs an RGB matrix whereby any other colour can theoretically be interpolated. But the colour space is now warped instead of being highly linear, as the scanner is without colour management. While the RGB primaries of my scanner probably don't match that of the Adobe RGB colour space that I assign to the scans, the colours nonetheless retain an accurate relationship between each other, and all I can say is they look much, much better. I'd be happy to link anybody to my Flickr page via PM so you can see what a home scanner can do without going through the pain of IT8 profiling.


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