Color Management/custom profiles at capture - slides

[nicnilov]

As you seem not to be an easily fend off type, and also because of your funny grandpa, let’s elaborate some bits.

@SSelvidge

Stacking them is something I had not considered, like stacking astro images. I will look into it. In that case is it expose for the highlights? Or perhaps shoot a bracket and combine, HDR style?

Bracketing could be useful and yes, the exposure should be for highlights. Depending on your camera’s ISO performance it may not be necessary to actually shoot several exposures. Would be sufficient to just convert the raw file with different exposure compensations and merge those. I’m not familiar with R5 but as a recent camera it should easily handle +2 stops compensation without making the noise any more apparent. Naturally, use the minimal native ISO when shooting.

Unfortunately, the issues of a high contrast input do not disappear with this approach. Diffraction doesn’t go anywhere with any kind of area sensor. Relatively even areas where the density differences are small should come out fine, but areas of high local contrast should be watched for flare spillover. The only truly efficient alternative is dot scanning, which drum scanners use. One thing to try would be to use a higher-than-target scanning resolution with a lens having higher than 1:1 ratio and then resize the image down.

This is where the rigs based on scanner lenses come into play. These have way better performance than consumer macro lenses, but often can’t capture the whole film frame at once, meaning stitching - another dimension of complexity.

@Digitizer

I’d give both lenses a try and would expect the Sigma to render more evenly.

Evenness does not need to be a factor. Use flat field correction both when preparing the profile and when shooting the slides. In addition to the lens vignetting this will also take care of any unevenness of the light source.

The Canon lens is fine, it should give around 4200-4600 ppi on the R5 sensor, which is likely sufficient. The Sigma one is likely sharper, but I wasn’t able to find any measurements.

@Harry

It is generally only dedicated scanner software such as Silverfast or Vuescan that can make use of the IT8, so-called “IT 8.7 or ISO 12641 compatible software”.

While it is true that IT8 transparency targets are typically used in traditional scanning, there is nothing that prevents one from using them with DSLR as we will shortly demonstrate. For making an ICC profile the target doesn’t need to be a particular one. ColorChecker, IT8, reflection, transparency, anything. One can take a sheet of paper, put some oil and acrylic paints on it in squares, and call it a target. As long as the patches were measured with a spectro, and the target layout description file exists, it will work fine. A profile can be created and will be very much more useful for images of the same kind than no profile at all.

@Harry

it certainly wasn’t straightforward though it was effective, and you’ll also find that the software required can be pretty expensive

The procedure itself is not difficult, but one does need to do the homework. The software is open source, free, modern, high quality, and comes with a decent amount of community support. One can get it here https://www.argyllcms.com, although this is not the only choice available.

@Harry

we’re not trying to reproduce the Mona Lisa.

Just to avoid further confusion, reproduction is a completely different problem domain with dissimilar constraints. What we all are mostly going for is a faithful interpretation of the look, which is already a very tall order if we consider the technicalities.

@Harry

Of course you’ll be shooting on RAW so provided the technical aspects are correct the colour can be sorted at any time.

Without calibration it really can’t.

@Harry

For absolute perfection you would need to remove the slides from their mounts and photograph them held completely flat, under Museum glass perhaps.

Flatness is a valid concern and it does need to be addressed. There are various approaches, e.g. ANR glass backing, all kinds of scanning frames, and the suggested focus stacking. Given that you (OP) already got a scanning rig, this issue should be familiar. Museum glass, as well as e.g. polarizing filters, are not advisable as these modify the spectral composition of light and, additionally, may interfere with the spectrum of the lightsource.

Besides, while 96-98 CRI lighting sounds reassuring, its worth it to measure the light composition with a spectro to be aware of the peaks/dips, and, more importantly, know which colors are the outliers. CRI rating calculation has important deficiencies which are rather relevant for scanning.

@DavidHoffmanuk

Just a point re the dense areas of transparencies or negs - these are far better reproduced with a camera based digitising setup than with a scanner. With a scanner the amount of light that can be squeezed through the dense area is fixed by the scanner beam speed and brightness and this leads to noisy results or lost data.

That mostly depends on the scanner and the software. The usual film scanner choice, the Epson family of scanners culminating with V850 and all the way back to 4990 in combination with VueScan allows modifying the (real, analogue) scanning exposure and offers multiscan (i.e. HDR). The deficiency of flatbed scanning is really not the exposure control, and not even the single-pass dynamic range. It is the low resolution (~2200 ppi) and the wait. That’s the primary reason DSLR scanning beats flatbeds. A ~45Mpx camera with a decent 1:1 macro lens achieves around 4600 ppi and is much faster, that’s all.

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Before we look at an example of DSLR calibration using a transparency target, let’s stop for a moment and consider all the steps one needs to perform in this seemingly fast way of scanning.

Preparatory steps

• Take a flat field capture (at the beginning of each scanning session)
• Prepare the ICC profile (once for a DSLR / lens / target / lightsource combination)

Scanning steps, for each frame

• Capture a normal single exposure
• Do focus bracketing (unless the alignment is good)
• Do exposure bracketing (unless after-the-capture compensation gives good results)

Processing steps, for each capture

• Interpret the RAW
• Apply the flat field correction
• Apply the calibration profile
• Convert to a working space

Processing steps, for each frame

• Merge the exposures
• Merge the focus brackets
• Save the master scan

Clearly, this all doesn’t look like something that should be done manually, especially when we are talking about “many thousands” of images. A meaningful automation seems to be a must, and that means scripting. That, in turn, means command line tools. This is another area where open source tooling beats commercial software pretty much every time. However, it also means more homework. This brings us back to the question of personal choice. Oh, well.

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Now for the promised example. Here is a capture of a transparency IT8 target on a 35mm slide, this is how ACR sees it. Note the black background and some details at the sides. The background is a piece of black cardboard with a 35mm hole in it, where the target is inserted. The details at the sides are where the frame area turned out to be bigger than the piece of cardboard, should have been avoided, but not critical. By the way, this was shot through a regular zoom non-macro lens, just for fun. That’s why the target area only takes a fraction of the frame. The backlight is ~D50 sunlight.

Using dcraw, convert the image to a tiff:

dcraw -v -T -o 5 -j -M -6 -W -g 1 1 -w -c chart-capture.nef > chart-xyz.tiff

Explanation of the options can be easily found online. The important bit is the raw gets converted with the linear gamma. Note how because of that the result looks much darker:

Then, for an easier target detection, I cropped the image. This is not strictly necessary, but makes things simpler:

Now take the measurements from the scanned capture. scanin is a tool coming with ArgyllCMS. Again, the explanation for all the switches can be found online.

scanin -v -dipoan -G 1.0 -p chart-crop.tif it8Wolf.cht F210418.cie

The it8Wolf.cht is the target layout desctiption file, comes with ArgyllCMS. The F210418.cie is the target batch measurements data file, available from the vendor of the target. The result is an .ti3 file, not yet directly usable. In the meantime let’s check the diagnostic output of the target detection:

Everything seems to be located nicely.

Finally, creating the ICC profile. This is a matrix profile, which is preferred over LUT profiles on small numbers of patches:

colprof -v -D"My Profile" -qm -am -u chart-crop

The profile gets placed to the appropriate system folder, Photoshop gets started up, and here is the image with the profile applied:

Similarly the profile can be applied to any other frame, scanned in the same conditions. Is this profile any good? Let’s check:

profcheck -v2 -k chart-crop.ti3 chart-crop.icc

> Profile check complete, errors(CIEDE2000): max. = 5.832972, avg. = 1.372846, RMS = 1.638548

For a matrix profile of medium quality on a limited number of patches (IT8 is not a big target) the results are quite fine. Subjectively speaking, the image could be brighter, but that’s down to the original target exposure, and also to the -u setting of colprof. This disables the automatic exposure correction by the profile, personal preference.

As you can see, the procedure is not excessively difficult, and in fact, is easily automatable.