Lens Bokeh Analysis

 

<<This topic is under development>> << Online version is only a draft >>

What characterizes the bokeh in a photograph? I mean not the shape of the bokeh, but the bokehness or the bluriness. We know that bokeh mainly depends on the aperture, focal length, and the focal distance. It also depends on few other things which will see later. The relationship between the size of the bokeh (circle of confusion) and the above mentioned parameters is a well established concept.

Here, I’d like to graphically plot and show how aperture, focal-length, distance and sensor size makes an impact on the bokeh. My objective is to provide a visual or a mental tool to quickly assess the blur characteristics given a set of parameters. And, perhaps help in understanding why bokeh in large-format photography is unique and often not reproducible in 35mm (full frame) format photography. We will also see how lenses such as EF 85mm f/1.2 and EF 200 f/2 differ in bokeh. Which of these lenses gives more bokeh? if you want to know the answer, please continue to read this discussion!

The size of the bokeh, let’s designate it by it’s diameter $C$, is given by

$C = \frac{\Delta }{{{s_1} + \Delta }}\frac{f}{N}m$

where, $f$ is the focal length, $N$ is the aperture, for instance $N=1.4, 2.8$ so on. $s_1$ is the distance to the subject, meaning, the camera is focused to the subject which is at a distance $s_1$. These parameters should be familiar if you are doing semi-serious photography. Now let’s look at less common terms: $m$ is the magnification and $\Delta$ is the relative distance. I will explain these two terms now.