Measuring the size of background blur

Many of us like to have good bokeh in (some of) our photos. Before we can obtain any bokeh at all, we need to blur the background. This post describes a simple experiment to measure the size of the background blur.

The obvious way to do this is to put a ruler in the frame and use the scale on the ruler to estimate the size of the blur (the ruler must be in focus):

The blur circles in this image have a diameter of approximately 25mm. Coincidently, the lens aperture also 25mm (45/1.8, i.e. the focal length divided by the f-number).

Of course, this is not really a coincidence. Simple geometrical optics can be used to show that the blur diameter will always equal the lens aperture, provided that the background is sufficiently far away (effectively at infinity). In the shot above, much of the background was around 20 metres away, which is far enough in this situation.

Taking another example, this time using a single small light source (a single LED) about 3 or 4 metres away:

In this case a more accurate measurement is possible: the blur disc is between 22 and 23mm diameter, slightly less than the aperture of 25mm. This is consistent with the light source being 3 or 4 metres away, which is only around ten times as far away as the ruler, so the blur diameter is about 10% smaller than if the light source was at infinity.

What about ordinary day-to-day photography? The photo below was taken at 58mm f/5.3, so the aperture was 58/5.3 = 11mm approx. It is easy to see that the size of the background blur is consistent with this by comparing it with the size of the girl's eyes (the human eyeball, including the white part of the eye, is very roughly 1” or 25mm in diameter).