How to determine the blurness of an image after using the canny filter?

Question

I would like to get a "blurness" value after using the cannyfilter on a siemensstar image, to check the blurness. Therefore I used the aforge lib to get these images after using the canny filtering:

Question: How can I determine now which image is sharper? Do I need to check the mean value of the histogram or..?

Thanks