What is bokeh?
This question splits up the photographic community in several groups. Overall you can say that bokeh is the not sharp area of a photo. For some photographers “good” bokeh is when the unsharp area is smooth and calm. Others love it when the bokeh is swirly (Helios 50mm f2) or bubbly (Meyer Optik Görlitz Trioplan 50).
Bokeh is created when light sources are out of the focus area of the lens. These light sources dont create a sharp spot on the sensor plane. They create a blurry spot shaped like the blur circle of the lens.
As you can see in this illustration the light from source A creates a huge spot because its in the out of focus area. Source C has the same phenomenom. Only light source B creates a sharp spot on the sensor.
If you open the aperture of your lens as wide as you can, the sharp area gets smaller and its easier to get bokeh in the background.
The shape of the bokeh is dependent to the shape of the lenses aperture. Lenses with a huge number of aperture blades like the Meyer Optik Görlitz Trioplan 50 create almost round bokeh bubbles.
Lenses with five blades in the aperture create a pentagonal like bokeh.
Thank you Ian Chattam for the example.
A different bokeh effect is the swirly bokeh of the Helios 44-M4 lens. It swirls the out of focus area around the center of the image.
In the end you cant say what bokeh is the best or perfect. It depends on what you want to create and what you personally like. Beauty is in the eye of the beholder.