a good friend has pointed me to this great test;
From Nick Paton:
This is a short test comparing a Canon 7d with an ARRI Alexa. The purpose of the test being to test the ability of each camera's images to be manipulated after exposure. I have graded each output to try to get it back to neutral using Gamma and density's chart. As you can see the Alexa responds very well where as the 7d footage fails in highlight exposure detail and its codec compromises its ability to be pulled up in the darker regions. It's interesting and telling and goes part way to explaining the more than substantial price difference.
Many have questioned why I have made such a test. I plan, at some stage, on having to intercut the two cameras in stunt or quick insert situations (the 7D) and wanted to know what I am up for. I also was interested in seeing what you get for your money. The Alexa is a known quantity and is known as one of the best digital cinema cameras currently on the market. Why not use a known quantity to compare the two. I plan to embrace either camera should either be the right tool for the job.