@rvietor and @Tamas_Papp thank you for such informative answers. So, if I understand right, the four-panel swatch on the scale card above should be used with a calibrated system and it can then be used to check, in post, that the colour values are correct - but not to adjust them? What does calibration entail in DSLR photography in this sense?
I have been pushing for ages for broader internal adoption of 24-panel colour checkers, at least in a lab setting and in a specific set of field settings where colour is important too, so hopefully this gives some more weight to my argument.
This all depends on what you know (control, assume) about your light source and the camera, the spectrum of your light source, the colors you are photographing (do any of them have a very narrow spectrum that has only partial overlap with the light source? this can happen with led-based lights not intended for photography) and how much you care about color reproduction.
Of course every single calibrated swatch adds some information. You could project into a subspace with some norm and minimize that… but you have to be very careful since (because of noise, etc) the system can be very ill-conditioned and you could easily end up with worse colors than simply doing nothing. I would not pursue this.
Given how cheap you can get a 24-swatch color checker, I think the most efficient use of your (and your coworker’s) time would be to just use one. Eg every time you photograph a bunch of objects, take a shot first for a specific camera and a specific light, then make a preset and apply that to all photos.
In darktable, you can just use a feature in the color calibration module, to create a module preset that’s correct for that camera under certain lighting conditions, as @Tamas_Papp mentioned.
Or more universally, you can create ICC profiles that can be used as input profiles in darktable or other software, although even these can need differences depending what software will be using them. I have used Lumariver profile creator, which is a paid version of the opensource DCamProf software, to create profiles for my main cameras. Incidentally, the Lumariver website has some quite good explanations on how it’s done and how it works…
Edit: There’s not much difference, technically speaking between these two methods - the all-in-dt approach has the benefit of being able to make new presets “on the fly” as it were, if you just take a snap of your colour checker whenever you end up with a new light source. It’s detailed fairly well in the darktable manual, in that page I linked above. Does need careful reading though…
Here is my question and I am happy to be shot down in flames by more knowledgeable people. If you matched the grey patch to neutral grey would not the other colors set themselves correctly? This seems to be the principle used in set a manual white balance in cameras. Yes I appreciate that color saturation would also come into play to get the correct ‘vibrancy’ to the colors.
So the images are already taken using this card? Did the team use different cards (ordered at different dates, kept in different location)?
Calibrations in general need a repeatable and traceable standard. If the images are taken using different cards, then you introduce an error (in this case color error), when you use card A vs card B. Card A could be from a different lot of cards with slightly different colors than B OR someone forgot about it in the car and it was exposed to UV light for some time, OR it is a 3 year card and the plastic degraded or the color degrade (they proudly describe the card as biodegradable) OR the card has dirt… I think you get the point. Unless the cards are controlled, old ones thrown away, you might not get consistent results.
I feel like your card which is perhaps printed on plastic?? which might yellow overtime?? Is really only good as an internal normalizing reference between photos and only likely for ones taken with the same card… I just quickly downloaded your image and maybe it was the export but if you balance on the white the grey was quite blue and if you balanced on the grey the white became much more yellow/warm… out of the gate the red green and blue were not badly lined up with those hues on the vectorscope but yellow was off and the saturation of green was considerably less than the other patches…not sure if that would be an issue but I can 't help but question the consistency of a 2 dollar card… So for me the best use case would be using it as an internal normalizing standard or reference that will not necessarily give you accurate colors but might give you a way to adjust a set of images taken with the same card to the same point…
Hey all, lots of great discussion here and useful answers, and I’m a newbie on the forum so I’m capped at how many people I can reply to directly, so kind of a general answer.
But essential, yes, @Claes is right - we use these cards in lab and field contexts for archaeological recording. I understand from all the answers that without a set calibrated setup (which is definitely not the case - we have several different models of field camera, and a couple of different lab cameras, and most of my colleagues use the cameras on Auto mode), so the best use of these cards for me will be as a reference tool - from what I understand, something I can check to make sure the images have been processed correctly.
For lab contexts, e.g. cataloguing and photographing large collections of artefacts, I have introduced my own setup which involves strictly controlled lighting, camera calibration, and a 24-panel colour checker, so there’s no real concern for lab imagery going forward. But as far as the existing archive of images, I understand I won’t be able to calibrate based on the four-panel cards.
@priort that’s not my actual card - just the photo from the webstore linked in the question. We have a large collection of cards printed to the same calibration spec on the same date that we can access, so that we don’t use damaged, faded, dirty, or miscoloured cards. But at the end of the day, since we work in very remote conditions and pack very light, we have (so far) been trading off the accuracy of top spec colour calibration equipment in favour of these.
Thanks everyone for so much information, it’s been very helpful! Og @Claes, kan du sende meg meldingen som du sendte til Terry, vær så snill? Jeg er interessert, og også en brukere på stackexchange har spurte de samme sporsmål og jeg ønsker å kunne svare (eller jeg kan gi jeg URL-en hvis du vil spurte!)
I agree that sharing the very informative message that you sent me would add to this conversation thread. I thought that white balance alone would preserve correct colors and you provide evidence that this is not the case.
You really aren’t going to get anywhere useful thinking about device dependent RGB and CMYK values. To actually profile a system you need to be dealing in device independent values, so that you have a basis for relating device dependent colors back to them.
i.e. first step is to measure the reflection spectrum of your reference card using a spectrometer. You then have some idea what these colors should be in some other defined colorspace such as sRGB or something else that you have a color profile for. Creating a useful conversion is a further exercise, but at least you would know what you are trying to convert from and to.
@gwgill So is it correct to assume from this that an Xrite/Calibrite ColorChecker has known values assessed by a spectrometer, and that in e.g. Darktable where they can be natively handled, the correct values are known for all available colourspaces?
I’m not a colour scientist or expert so this is possibly a bit of an ignorant question, but if a printed colour has a known CMYK value, is that not the same as knowing the value from a spectrometer? I have access to multiple spectrophotometers (is that not what you meant, rather than a spectrometer?) so I can check the colour values - but the outputs from a spectrophotometer are RGB, CMYK, hex, etc. which I already know for this card. How is that different?
Very, very far from the same thing. CMYK values are the notional amount of ink laid down. What color that actually makes depends on the nature of the inks, the way they are screened, the paper they are printed on, and the light being used to illuminate the print. All of that is highly variable with every component involved. Similarly with RGB values. They are device dependent - i.e. the color they produce depends on the nature of the device used to display them.
The raw output of a spectrometer is the percentage reflectance at each wavelength. Add an illuminant (say D50 by default) and an observer model (say the 1931 2 degree standard observer) and you can compute the device independent XYZ values that represent the color. That’s the common “language” to connect device dependent colors.