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
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.
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