blurring the foreground

I read in a tip in a Canon brochure which showed how to make an obstacle disappear from a photo. The example in the brochure, which was advertising a very expensive lens, was of someone attempting to take a picture of a tiger at a zoo that was behind a fence, then using an in-camera trick to make the fence disappear and have the resulting photo look like it was taken in the wild.

The trick really is about setting the aperture (f-number) setting on the camera to restrict the depth of field.

First, a description of the scene and setup. The photo below is an entrance doorway to Eastern Market in Washington, DC, recently gutted by fire. I was hoping to take some photos of the damage and repair work in progress, but the doors happened to be closed and locked—most likely for safety reasons. Note that the door has metal mesh as a window. The goal of this test is to see whether or not I could blur out the mesh window and as a result, determine if the example in the brochure was legitimate.


Eastern Market Entrance Door, with Mesh Window

I set my camera to aperture priority mode. This means that the camera meters the scene (takes a light reading), I choose the aperture (f-number) setting, and the camera calculates and sets the correct shutter speed to obtain a good exposure. One can do this in manual mode as well -- just look at the light meter and when you set the aperture, adjust the shutter speed to match the same position on the light meter each time, assuming you meter the same way each time. The focus is set to about halfway back in the market, about where the DC flag is on the right.

Starting at f/8, I started stopping down to f/7.1, f/6.3, f/5.6, without noticing much of anything happening. (The results are only visible in the resulting photograph as the aperture setting takes effect when the shutter is released.) The effect started showing as I stopped down further.


Mesh at f/5.6 for 1/8 sec


Mesh at f/3.5 for 1/25 sec


Mesh f/2.8 for 1/40 sec


Mesh at f/1.8 for 1/100 sec


Mesh at f/1.4 for 1/160 sec

So, it sort of worked. You can still see some of the green mesh with the aperture set wide open at f/1.4. But it's nowhere near as distracting as in the f/5.6 photo. Given the green color of the mesh and the objects visible behind the mesh, this is probably as good as one can expect it.

When the aperture is opened wider (by reducing the f-number) the depth of field becomes more shallow. The depth of field in a photograph is defined as the range of object distances within which objects are imaged with acceptable sharpness. By focusing far enough back in the room and then reducing the depth of field such that objects in front fall out of the depth of field, the objects will become not acceptably sharp and one can blur the foreground into the scene.

(I need a 3D diagram and some equations and numbers to illustrate this better.)

If the mesh was neutral-colored enough and thin enough (maybe like chicken wire), and the background was busy enough, I think one could make the mesh "disappear" from the scene.