It’s because the spectrum of light output by the iPad is so different from the light that was used to create the camera profile. if you think about an iPad screen, there are no white pixels, only red, green, and blue and the most recent iPads (those advertised as having a P3 gamut) have very narrow spectral peaks each color:
Compare that to the spectrum for daylight, which camera profiles are based on:
This is a VERY good thing for scanning film because it works with the camera sensor to create a spectral response that is much closer to film scanners and the photo print paper that they are designed to emulate. For instance, here’s the spectral output of the Fuji Sp3000’s light source:
Notice the gaps between the R/G/B channels. This is imperative for good scans because those gaps are the wavelengths where the infamous “orange mask” is stored. Scanners and photo paper are designed to have extremely low sensitivity to yellow/orange wavelengths of light in order to filter out the mask, but camera profiles expect a very high amount of yellow light, because that’s actually what camera sensors are most sensitive to.
In the image below notice how the cameras red sensitivity peaks at the exact point where photo paper’s sensitivity is lowest. This is the crux of why people have so much trouble getting good color with camera scans. Because the camera is so sensitive to yellow/orange light it is absolutely critical to eliminate the yellow/wavelengths from your light source, either by using a light that doesn’t emit them, or by using a special kind of filter that removes yellow/orange but passes red light. Normal dye based cooling filters remove yellow, orange, AND red, which is the same thing that digital white balancing does.
The iPad screen is a wonderful light for scanning, as are most of the RGB Leds on the market today. Normal LEDs, even the high CRI options, aren’t ideal because they actually emit more yellow light than they do red light. When you combine this with a camera that has more sensitivity to yellow than it does to red (and in multiple channels!) it becomes impossible to get good color without individually tuning the HSL balance of every single image and even then tones will still be muddy and off.
You really need to use a scanning light source that emits as little yellow light and as much deep red light as possible. Here’s an overlay showing the differences in spectral sensitivity between a digital camera, photo paper, and an ideal film scanning light source. Note that the spectral peaks of the photo paper response are actually much narrower than they appear here because they are plotted on a logarithmic y-axis while the camera sensitivity is plotted on a linear y-axis.
The reason that Rec2020 looks so much better is that (1) it is more similar to the gamut of the iPad’s native DisplayP3 color space. When you’re using a backlight with narrow band primaries the backlight determines the color gamut of the image rather than the camera sensitivity primaries as would be the case when photographing with full spectrum light. The goal then is to use an input color space that reflects the gamut of light output by the backlight. Secondly, REC2020 is also closely related to the color gamut of photographic negatives and print material, so the colors of the negative are being mapped very closely to where they are supposed to be. Here’s Rec2020 (white translucent frame) compared to Kodak motion picture print film, which is very similar to photo print papers.
It’s also a very close match to the gamut of this Kodak motion picture film:
Compare that to ProphotoRGB, which most camera profiles are based on, and you can see how the primaries aren’t lined up nearly as well.
Anyway, as far as white balance goes, the ideal method for film scanning is actually to turn off Darktable’s white balance module entirely (or set R,G, and B all to 1.0) which will normally give a very green image) and use an app on the iPad (like Color Savvy) to adjust the RGB output until the unexposed part of the film leader (or image border if there isn’t flare leaking into it from the image) is the same for all three channels and within 1/3 stop of clipping . Write down the values for each film stock that you scan so you can set them quickly next time.
If you really want to go crazy, you could try using a print paper profile as either the working space or gamut clipping space. I haven’t done this yet, but I think it might go a long way towards getting really film-like color. Try one of these:
Fuji Frontier Scanner working color space:
Fuji_Frontier-PD_CA-HD_v3a.icm (175.6 KB)
Fuji Crystal Archive Photo paper profile:
Fuji_Frontier5-sRGB_CA-DPII_v3a.icm (175.6 KB)
Kodak 2393 and 2383 print film profiles:
FilmTheaterK2393PD.icc (391.1 KB) FilmTheaterK2393PD.icc (391.1 KB)
Hope that’s helpful!