Many new to photography use the term bokeh incorrectly.
They use the word bokeh when they are really asking about, or discussing depth-of-field (DOF).
Depth-of-field is quantitative and is affected by:
- image sensor size
- lens focal length
- subject to image sensor distance
- subject to background distance
- lens aperture
There are DOF calculators online. You can plug in values for the different variable listed above, and the DOF calculator will describe the resulting DOF:
Online Depth of Field Calculator
Bokeh is qualitative and mainly a function of the number and shape of the blades that are the lens aperture. More blades, rounded blade edges, and blades shaped to make an aperture opening as close to round as possible, usually produces the softest bokeh.
There are 2 main types of bokeh:
- Cream cheese (very smooth)
- Hollywood (nice round blotches of light)
A good example of a lens that can produce very high quality bokeh, is Nikon's AF 85 mm f/1.4D lens. It has been nicknamed the 'Cream Machine'.
The Cream Machine has 9, rounded edged, and carefully shaped lens aperture blades.
An example of a lens that produces poor quality bokeh is Canon's inexpensive EF 50 mm f/1.8. Canon's 'nifty-fifty' has only 5, sharp edged lens aperture blades.