timbop wrote in post #17250763
So they don't really test color ACCURACY, unless I misread their attempt to confuse and befuddle anyone by displaying some intentionally important-looking formula paired with a bit of terminology from Isaac Newton. In the end, it seems they just compute the number of bits required to store calculated values for range - which isn't really the same thing as reproducing the colors that went into the front of the optics. But who can tell?
Color "accuracy" isn't something that even has meaning in the context of a raw image. Bit depth of the colors recorded by the sensor, on the other hand, does.
When you think about it for a moment, it makes sense that such is the case, but it requires a solid understanding of how the camera actually captures an image. And so, I'll explain.
The camera's sensor is a matrix of light sensors covered by a bayer pattern filter:
When the camera is recording a color, what it's really doing is recording the intensities of the red, green, and blue components of the incoming light. That's all it does
. The camera readout electronics convert the recorded light intensities into digital values and that is what goes into the raw image. The raw image contains the color component intensities.There is no such thing as color "accuracy" for this setup
. Instead, what you have is a translation matrix that converts the recorded values into the colors we see. The process of converting the bayer pattern image into a color image is called "demosaicing", and final color conversion clearly has to involve the use of an additional translation map. It is the accuracy of that translation map that determines the accuracy of the resulting colors
Your raw processing program does the demosaicing of the recorded image and applies the translation matrix itself. It is your raw processor, not
the camera, that has the translation matrix. As such, any color accuracy errors are the result of errors in the translation matrix, not
in any deficiency of the camera. It is for this reason that things like the Colorchecker Passport exist: they allow you to create a color translation profile for your camera and lens combination (yes, I did
include the lens, because the lens does
have an effect on the resulting color intensities recorded by the sensor).
If DxO were to test the color accuracy of the camera, what it would really be doing is testing the color accuracy of the JPEG converter inside the camera. While that might be useful for some, it doesn't really say anything about the sensor's
capabilities, and it is the sensor's capabilities (more precisely, the capabilities of the data recording pipeline that starts with the sensor and ends in the raw file) that DxO measures.