bokeh

introduction

bokeh is the subjective aesthetic quality of the appearance of out of focus highlights (OOFHs) in a photographic image
aesthetic bokeh is generally when out of focus areas blend smoothly with each other
nervous or busy bokeh occurs when the lens creates harsh, sharp lines around out of focus subjects, especially specular highlights, and this results in any thin lines in the OOF area being transformed from a single OOF line to TWO extremely SHARP lines with some blurring between them
some subjects are reknown for giving nervous, busy bokeh when out of focus, and these should be avoided whenever possible:
- specular highlights including sky between leaves
- tree branches
- long thin leaves
- chain link fences
as it is largely dependent upon the spherical aberration characteristics of a lens, the foreground bokeh is usually quite different to background bokeh for a given lens and bokeh also changes with relative subject distance and background distance and an experienced photographer who knows his lens will work this to their advantage and avoid less optimal situations.
factors which affect bokeh:
- spherical aberration determines how evenly the light is distributed within the OOFHs with lenses with busy looking OOFHs (poor bokeh) having OOFHs with strong peripheral double contours instead of a single contour
  - over corrected spherical aberration results in prominent bright outer rings in the background and this can result in Nisen Bokeh when photographing certain backgrounds
  - under corrected spherical aberration results in prominent bright outer rings in the foreground
  - in other words, designing a lens for nice foreground and nice background bokeh is a compromise unless you add in variable bokeh control (this is available on some specialised 135mm lenses)
  - onion-ring bokeh is the result of modern Precision Molded Optics (PMO) technology, a technique used to make aspheric lens elements
    - Panasonic has invented a new lens manufacture process that uses a novel method to remove onion bokeh which has been a problem with modern aspherical lenses - see http://www.imaging-resource.com/news/2014/05/02/the-end-of-onion-ring-bokeh-panasonic-beats-the-curse-of-aspheric-lenses
- shape of the aperture diaphragm partly determines shape of the OOFHs
- mirror lenses have a central obstruction (the secondary mirror) which causes characteristic donut shaped OOFHs
- optical vignetting may also create oval shaped OOFHs (Cat's eye effect) in the peripheries which can produce a rotatory effect with the ovals being aligned in a circular manner around the centre of the image - this is particular likely in certain lenses at wide apertures and short focal lengths but is even present on 85mm f/1.2 lenses
- clipping of OOFHs by internal obstructions such as a mirror chamber that is too small
- transverse CA may cause colour fringing of OOFHs with magenta on one side and green on the other side of the OOFH
- longitudinal CA may cause colour in the OOFHs
- astigmatism may distort the OOFHs and give oval shaped OOFHs pointing towards the centre

lenses known for their smooth background bokeh

Rokinon 85mm f/1.4
- see http://www.flickr.com/photos/44919156@N00/sets/72157627312850608/ for image examples on Micro Four Thirds system