The 135 STF Design
Smooth Transmission Focus
While subjects remain in sharp focus, out-of-focus backgrounds or foregrounds are blurred smoothly and evenly with gradation that thickens toward the edges of the frame for a pleasing aesthetic effect.
This site reviews the design, handling, and concepts underpinning this fabulous lens.
The unique lens in the range 135 mm F2.8 [T4, 5] STF produces superior image quality with incredibly soft and natural blur characteristics. Through the installation of an apodisation filter, points closer than and further than the focal points appear equally soft and transition smoothly from the focal point. With conventional lenses the fuzzy points are depicted in clear circle patterns.
In a conventional lens, the amount of light collected at the periphery of the lens is roughly equal to the amount of light at the center. This results in uniformly sharp dots
at points b and c, below. The STF lens, however, uses a special filter called a "apodization optical element" that collects less light at the lens periphery, which results
in diffusion at the edges of the dots instead. Smoother defocusing is due to this optical characteristic.
(1) Apodization optical element
(2) Defocusing of STF lens (around focus point "a")
(3) Defocusing of conventional lens (around focus point "a")
The unique "apodisation element" is situated near the aperture of the lens optical system. This special optical element is a type of ND filter which gradually becomes thicker (darker) towards the perimeter, thereby reducing the amount of light that passes through around the outer perimeter.
The 135 STF has two apertures.
- In manual aperture control mode, it's a 10-blade aperture with rounded sides.
- In A (auto) mode it's Minolta's "standard" 9-blade aperture with rounded sides.
The apodisation filter is a lens element - a unique and integral part of the lens design. It absorbs 1.5 stops of light. Thus the lens is limited to being a 4.5 lens in light transmission. However, the design doesn not impact on the depth of field, which is normal of a 135mm lens with an aperture of 2.8.
This is the basis for the formal designation of the lens as f 2.8 [T4.5].
The design is entirely unique in photography.
The closest immitation is offered through Nikon with their 135/2.0 AF DC (Defocusing Control) Nikkor lens, designed with similar purposes. The Defocusing Control is an excellent lens, that offers Nikon users the ability to make out-of-focus areas appear softer by strategically introducing spherical aberration to just the defocused areas (in-focus areas are not affected). The Nikon DC lens has Autofocus, but cannot meet the supreme standard of bokeh of the STF lens... the DC lens shifts lenses around to over-correct or under-correct for spherical aberrations, to optimise the unsharp areas in either the foreground or background, but not both. Only the apodization filter in the STF lens optimizes both foreground and background unsharp areas.