Background Blur ... It's Not Just Focal Length and Aperture

There's something about shadows that attract my attention.  More particularly, the lines that define the boundaries between light and shadows fascinate me. When created by sunlight and its reflections, they are fleeting and constantly in transition.  I barely have enough time to grab my camera and check the ISO before the composition changes in front of me.

A few days ago the morning was warm enough for me to leave the door open.  While the sun was still low enough in the sky to cast beams of axial light, this form appeard on the screen door.

The camera I grabbed had a 100mm macro lens on it.  I got up close enough to capture the detail of dust the sunlight revealed, shooting at f/2.8 to completely blur the background.  This close, the 100mm lens rendered everything behind the screen a milky smooth cloud.

I pulled back for the next image.

The frame includes the sunlit screen on the lower left portion, and through the shadowed portion of the door is the outline of a stucco arch on the porch.  Foliage is in the background. 

As the second image illustrates, stepping back a couple of feet, even with the lens still wide open at f/2.8, makes a big difference in what happens to the out of focus background. 

For background blur, it's all about ratios.  One foot in front of the subject, with a background 30 feet behind, is a ratio of 1:30.  Step back two feet, and the ratio drops to 1:10.  This threefold reduction brings out some things that were just completely obscurred when the ratio was 1:30.  It's not just the focal length and the f/stop that affect the background blur, but also the subject to camera distance compared to the subject to background distance.