log C encoding

[miniproducer]

Hello,
i am reading about gamma correction, i've found that some cameras (ARRI) use log C encoding.
can someone explain the follwing image?

taken from
http://www.arri.de/camera/digital_ca...709_video.html
why does the 18% level seems draker and darker? if it's 18% luminance it should be higher (bacause the maximum always gets higher).
what is the exposure (in stops)? do the mean that they measure luminance in exposure? i know that exposure is logarithmic, right?
why do they map the 18% to code 400? i understand that 18% should represent 0.5 in linear scale.
thank you.

[GrahamS]

I think that you are in the wrong forum, but this may help you:
http://www.arri.de/camera/digital_ca...709_video.html