Bokeh is about what happens away from the point of best focus. Photographic lenses were originally just designed for sharpness within the possible depth of field. The qualities of the out of focus areas were just incidental by-products of lens designs.
Initially in Japan, and later throughout the world, interest developed (excuse the pun) in the qualities of the out of focus areas. The Japanese word ‘boke’ (roughly translated as ‘fuzzy’, spelled ‘bokeh’ to guide pronunciation), has become the standard term used to describe the smoothness of the out of focus areas produced by a given lens.
Most photographers who care about bokeh prefer the out of focus areas to be smooth and soft, and most of all to not be distracting. As a general rule, prime lenses (fixed focal length lenses) tend to have better bokeh than zooms. Macro lenses and telephoto lenses tend to have better bokeh than standard lenses. Wide-angle lenses using retrofocal designs tend to have poor bokeh (e.g. lenses below 50mm). Reflex lenses (also known as mirror lenses) tend to have very distinctive ring or ‘donut’ bokeh, consider very distracting by some.
Minolta's approach to bokeh
Minolta, along with Leica, was a leading manufacturer in the design of lenses deliberately designed to produce pleasing bokeh. Other manufacturers tended to focus (again, excuse the pun) more on pure sharpness – on achieving optimal MTF results. As bokeh is more subjective than sharpness, it cannot be so easily quantified or measured on standardised lens tests (though on the MTF page I describe ways to read MTF charts that may suggest good bokeh).
Minolta pioneered bokeh research and created several special lenses with enhanced rendering of out of focus areas (though Nikon later developed some lenses with more deliberate attention to bokeh, such as their 85mm f/1.4). Designing lenses to render pleasing bokeh should not be confused with Soft Focus lens designs – another innovation pioneered by Minolta’s lens engineers.
Spherical aberration and the circle of confusion
Spherical aberration is when the rays of light from the middle and from the outside edges of a lens do not focus to exactly the same point. Differing amounts of spherical aberration alter how lenses render out-of-focus points of light, and thus their bokeh. A technically perfect lens has no spherical aberration. Therefore a perfect lens focuses all points of light as cones of light behind the lens. The image is in focus if the film is exactly where the cone reaches its finest point. The better the lens, the tinier this point gets.
If the sensor (whether it’s film or digital) is not exactly where that cone of light reaches its smallest point, then that point of the image is not in focus, instead, that point is rendered as a disk of light -- a "circle of confusion". A lens with no spherical aberration produces circles of confusion that are evenly illuminated discs, either with hard edges or minor halos caused by optical diffraction. These disks result in unpleasant bokeh, as the harsh circles can be distracting and give a false sense of definition to elements of the image intended to be out of focus.
Bokeh control: The 135 STF
The 135 STF offers a unique level of control over bokeh, thanks in part to the apodisation filter placed next to the aperture. This element does not appear in any other lens design on the market, past or present. By gradually darkening towards the edges, the apodisation filter operates as a translucent layer between the aperture opening and aperture blades. The effect produced is circles of confusion that gradually dissolve into the background, creating a smoothness that is impossible with conventional lenses.
Bokeh changes for different apertures. As you close down the aperture the edges of the lens are cut off from the light path thereby removing the spherical aberration problems that may be affecting the Bokeh. Because the 135 STF provides stepless aperture, allowing continuous adjustment, rather than being constricted to a series of ‘stops’, the nature of the bokeh can be controlled to an astonishing degree.
The 135 STF has arguably the best bokeh of any lens produced for Single Lens
Reflex (SLR) cameras Of course, there is no meaningful way to quantify this, so
the statement is highly subjective, but I'd say very few users of the 135 STF
have ever felt that it's bokeh performance was anything other than