Incorrect Densitometer Readings

Question

I am running Aperture 2.0 and recently did a greyscale test on a Canon 40D using an 18% grey card as the control.
The RGB densitometer readings (using the loupe) were largely as expected when reviewed with the 18% grey card falling in Zone 5
There are effectively 10 stops of dynamic range (Zone 0 to 9), and Zone 5 falls approximately in the middle of this dynamic range both as a numeric value and as a step in the greyscale.
There does seem however to be a notable anomaly in my results, as depicted in the spreadsheet below:
ZONE R G B
Zone 0 20 21 20
Zone 1 25 25 25
Zone 2 28 31 31
Zone 3 44 46 47
Zone 4 67 68 68
Zone 5 113 114 115
Zone 6 163 165 165
Zone 7 207 210 209
Zone 8 246 247 247
Zone 9 255 255 255
HOWEVER, an anomaly occurs in Zone 0 to 2 as follows:
1. In Zone 0 to 2 the RGB values seem squashed into a narrow RGB band (effectively 20 to 30)
2. As ridiculous as this sounds There seems to be no “pure black” zone in my grey-scale test.
I know it is not the camera as if I run the same test in the canon software I get more correct RGB readings with the final zone reading 0,0,0 for the RGB values, which is effectively pure black
Any ideas?

Answer 1

Hear is one idea - maybe different RAW processors process RAW files differently with different tone curves. Maybe that's what is causing this unheard of effect.
RB