Bokeh is the pleasing defocused area that draws your attention to the subject, and avoids the distractions of the other areas. While the AMOUNT of Bokeh is hard to quantify, and can vary even with the same equipment with the subject and background at different distances, the potential amount of Bokeh is quantifiable.
The formula is
((Focal Length/minimum f-stop)/minimum focusing distance)/crop factor.
So the factors that will affect bokeh are
1) focal length- the longer the zoom, the more bokeh available
2) aperture- with a wider aperture (lower f number), more bokeh is available
3) the closer the subject is to the camera, the more bokeh is available. A subject 20 feet away will not provide bokeh the way a subject 2 feet away will
4) with a full frame camera, there is more bokeh available. When they discuss the relative merits of full frame vs digital size DX sensors, this is the one factor that is often overlooked. At any given zoom, there is more bokeh on the bigger sensor.
For example, using a 50mm lens f.14 lens with a minimum focusing distance of 1 foot and a crop factor of 1.5 (such as the Pentax 50mm f1.4) would yield a number of ((50/1.4)/1)/1.5= 23.8. Note that on a full frame camera, that number rises to 35.7 or one and half times more bokeh is available. This is why some photos taken on a full frame camera cannot be reproduced on a DX (smaller sensor) camera, and photos taken with the $5000 Nikon 200mm F2.0 can't be reproduced with the equipment that I have.
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