Hi Dave,
That's only partly true. Metamerism is something that is caused by the specific spectral transmission characteristics of the Bayer CFA filter layers and the spectral properties of the color it samples. This will cause two subjects with the same colors to be interpreted differently depending on the illumination and reflection spectra when shooting that subject. A color e.g that has spectrally pure yellow color, and a color that is a mix of red and green light, may look to a human observer like they have the same color, but the camera may see a difference (a 'wrong' color representation).
Skin color is e.g. rather difficult, because it reflects a mix of reds and yellows and Infra-red that locally varies as relatively more or less blood is directed more to the surface or more to deeper tissues, and pigmentation and skin thickness add more variation.
A profile as such will be able to reduce (but not totally eliminate) all of the errors, but usually only for a specific illumination.
The Bayer CFA filters are also not spectrally perfect filters, they have secondary absorptions and transmissions, which will contaminate colors with different wavelengths than the R/G/B pass band that is supposed to be sampled. For example Blue light will register as a bit of Red, and Infrared light will register a muddy brown because it also adds blue and green signal, thus desaturating and changing the other colors. This will not be separable after the pollution has taken place. Just like you cannot unscramble an omelette.
Cheers,
Bart
So if a weaker CFA is used to let more light into the camera (so ISO can be increased without increasing noise), from what I understand, colors will be less differentiated and less accurate, and that can't be changed with computer software, because "you cannot unscramble an omelette?"
Is that true?
And if the colors in the CFAs are altered for the camera to perform better in artificial light, we can try to counter that in post, but what's done permanently in the CFAs is probably hard to change?
And the same would be true for how they tweak the CFAs to get "more pleasing" skin tones; even though, this would not help landscape colors?
It's hard to find any hard evidence proving this, but
this Luminous Landscape article first alerted me of the weakened color filter array problem, and that helped explain to me what was going on with my 5D3 (74 metamerism score), which had color issues that I didn't notice on my 5D2 (80 metamerism score).
[[[As one example, the selection of a CFA, the color pattern put in front of the sensor, is a
choice between quality of color, and ISO performance. If the CFA allows each pixel to see a broader spectrum of color (e.g. for the
green pixels to see a bit further into yellow) a camera’s ISO range can be modestly increased. The resulting loss in color quality is subtle –
subtle variations in color are missed and a handful of specific colors become difficult to photograph.
In a market where a ISO 25,600 camera has a leg up on a ISO12,800 camera, the engineers are under enormous pressure to pick the modestly increased ISO over subtle color quality.]]]
This article is also interesting, and apparently Sensorjen derives his QE numbers from DxO data. Not sure.
[[[However, it is not merely the total amount of light that falls on the sensor that determines the noise in the photo, but also how efficient the sensor is. The primary attributes of sensor efficiency are the
QE (Quantum Efficiency -- the proportion of photons falling on the sensor that are converted into electrons) and the read noise (the additional noise added by the sensor and supporting hardware), although it should be noted that a manufacturer may use a weaker CFA to increase the QE, which will reduce the luminance noise at the expense of increasing chroma noise. For example, two sensors may have the same QE and read noise, but one sensor may have a weaker CFA than the other, making it a more noisy sensor.
Sensorgen is an excellent resource for sensor QE and read noise, although it does not give information about the CFAs.
However, while QE and read noise are the primary attributes to sensor efficiency, there is more to the story, such as the Bayer CFA (color filter array) which most all cameras use. The Bayer CFA is a color filter covering the sensor in an RGGB pattern. This means that 25% of the pixels are covered with red filters, 50% are covered with green filters, and 25% are covered with blue filters. Of course, the filters actually accept a range of colors, which overlap (otherwise, yellow photons, for example, would never make it through the color filters). How much the filters overlap, and how strong the filters are, also contribute to the sensor efficiency.
For example, the green filter may only admit 60% of the green light that falls on it, but also admit 10% of other colors that fall on it.
If we use a weaker filter to increase the transmissivity, we will reduce the luminance noise (more total light will pass through the filter and onto the sensor),
but also increase the transmission error by concomitantly
allowing a greater percentage of other colors to also pass through. Thus, different manufacturers may strike different balances between luminance noise vs color noise in their choice of color filters. For example, it has been argued that
Canon has been steadily "weakening" their CFAs to increase the QE of their sensors. The 5D has a QE of 25%, the 5D2 has a QE of 33%, and the 5D3 has a QE of 49%, as the metamerism index has steadily declined from 84 to 80 to 74, respectively.]]]
I had always thought that Canon would never mess with low ISO performance shot in natural daylight (CRI of ~100), but it looks like I was very wrong.
Canon isn't saying, and few seem to understand what they've been doing. I don't know of even one article that addresses this topic only.
