Why is my far away background in focus even with a low aperture number?

Question

I took a picture of my son in an elementary school cafeteria. The room was very well lit and the parameters of the shot were Canon T3, 18mm, F/3.5, 1/125, ISO 100. I was close to him and his head basically filled the frame. He was about 20 feet from the wall and against the wall there were tables with books on them.

When I took the picture, he was totally in focus but the wall, table, and books were also in focus, though not as sharp as he was. I expected to get a very blurred background and was surprised that I could make out everything 20 ft behind him in such detail.

Does anyone know why the shot ended up that way? Thanks!

Answer 1

The small degree of defocus in the background is due to the focal length used being very short (18mm).

The amount of background blur depends on the size of the entrance pupil, not the f-number. The entrance pupil size is the focal length divided by f number, so in this case it would be about 5mm. This is quite small. A 100mm lens at f/3.5 would have an entrance pupil of size 29mm.

For this reason you will get more out of focus backgrounds at the other end of the kit zoom range at f/5.6, at 55mm the entrance pupil will be twice the size at 10mm.

Answer 2

I am guessing that you have an 18-55mm. In this plot the background blur between 18mm f3.5 and 55mm f5.6 are compared for a head and shoulders portrait. As you see the background blur at 6m (~20ft) is almost twice as much at 55mm f5.6. The entrance pupil mentioned in a previous answer is only valid when the distance between the subject and the background reaches infinity.

You can see this comparison here. In general there are three factors which give more background blur for a given subject framing:

1. Wider aperture
2. Longer focal length
3. Bigger distance between subject and background

Answer 3

The DOF depends on the ratio between the distance you put the focus on, and the focal length; and also on the numeric aperture.

You used a low aperture; it helps to keep a low DOF. Buy if your point of focus is, say, 5 meters away and your focal length is 18mm, be sure it is enough to keep in focus the whole scene behind 5 m distance, even with so low aperture number.

Answer 4

There are two components to blurring: one is the size of blur disks in the world: for a given subject filling the frame, that solely depends on aperture number and the multiple of the correct distance that the background is away.

However, the size of the blur disk depicted in the photograph additionally depends on perspective.

The size of the depiction shrinks with the inverse of the distance from the camera, the size of the in-world blur shrinks with the inverse of the distance from the in-focus plane. So they balance out eventually regarding their effect on the photograph. This balance is achieved earlier for smaller focal widths and consequently smaller subject distances (given the stipulation that a subject is to fill the frame).

So while one would think that having the subject much closer and reducing focal width to accommodate the framing should increase background blur, this does not actually work in that manner because while the background gains additional blur, it also shrinks so much perspectively in relation to the subject that the blur does not offset the gain in details because of the small size.