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Subject: Re: ACR Calibration



-->Obviously, varying exposure adjustments were needed to arrive at these values.

Therein lies part of the problem. The numbers WILL vary with exposure.

For example, here are the results for three different exposures from my 300D, with Exposure and Brightness adjusted to get the white patch to around 245 and the green value in the green patch to 124.

CRW_0728

Red = 132 61 48
Light Skin = 173 148 127
Green = 79 124 61
Yellow Green = 149 177 72
Blue = 44 35 116
Blue Sky = 92 103 141

CRW_0729

Red = 134 60 47
Light Skin = 174 148 128
Green = 78 124 60
Yellow Green = 152 180 72
Blue = 41 31 113
Blue Sky = 92 103 142

CRW_0730

Red = 134 62 49
Light Skin = 168 144 124
Green = 79 124 62
Yellow Green = 151 180 73
Blue = 39 30 110
Blue Sky = 91 101 141

The values are reasonably self-consistent in that they're more like each other than they are like the values from your camera. I'm fairly confident that the differences are entirely due to the exposure settings, but I don't have any way to prove that it isn't also due to variations in the lighting.

All 5200K strobes are not created equal. 5200K is a correlated color temperature, and many different spectra can produce 5200K white. Are the differences between your 300D's blue channel and mine due to camera differences, or to differences in the spectra? We don't have the data that would let us make that call.

My experience strongly suggests to me that unit-to-unit variation is a real issue. But proving it isn't a trivial exercise.

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