Here is a 7D ISO test I did. Hope it can show you that when shooting with the 7D it is preferable to shoot at ISO 160 or 320 and only when necessary above 400. The reason why 100 is more grainy than 160 is because (to my understanding) that ISO is not a native to the sensor and it's sensitivity algorithm. My process was to take pictures with the front cap on "black shading" the camera at different ISOs. Then I increased the exposure in Photoshop to pull out the grain and what you see it the full images next to their magnifications.