Out of Focus
Normally when photographers are testing there lenses their often only have an eye on the sharpness - but the most of what we picture is out of focus - foreground, background. Think the focus level in the pictures as a thin glass plate you move forward and backward. The higher f-stop - the thicker glass plate.
I have started to test the look of defocus by the lenses - how they blend contours, colors and contrast. I even started to take pictures completely out og focus. It is very inspiring.