Minolta STF 135mm f/2.8 T4.5 lens

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The correct title of this article is Minolta STF 135mm f/2.8 [T4.5] lens. The substitution or omission of any < > [ ] { } is because of technical restrictions.
Minolta STF 135mm f/2.8 [T4.5]
Minolta-STF-02.jpg
Key features
Maker: Minolta, Sony
Ultrasonic motor: No No
Macro capable: No No
Unique features: Smooth Trans Focus
Application: Smooth Bokeh
Technical data
Type: Special Prime
Focal length: 135mm
Aperture (max/min): f/2.8 [T4.5] - f/32
Construction: 6 groups / 8 elements
# Diaphragm blades: 9/10 circular
Close focus distance: 87 cm
Max. magnification: 1/4
Physical
Weight: 730 g
Filter diameter: 72 mm
Accessories
Lens hood: Bayonet
Angle of view
Horizontal: 18.2°
History
Introduced: 1999
Replaced by: Minolta version succeeded by Sony version in 2006
Retail info
MSRP US$ $1200 USD (Sony version)

Originally produced by Minolta, and currently produced by Sony, the STF 135mm f/2.8 [T4.5] is a photographic lens compatible with cameras using the Minolta AF and Sony α lens mounts. STF stands for Smooth Trans Focus, in reference to its special optical system, which is intended to smooth the transition between the plane of focus and out-of-focus areas in the image. It is not a soft-focus lens.

The STF is a manual focus-only lens, due to the use of an apodization filter that provides the high-quality bokeh effect. This lens is the only genuinely Minolta/Sony A-mount lens produced without autofocus capability. It is also the sole such lens having a manual aperture ring.

Apodization is a process in spatial signal processing which can enhance resolution by reducing the secondary maxima in the diffraction pattern of the lens' aperture. The STF lens features an optical apodization filter in form of a neutral-gray tinted concave lens element near the lens' diaphragm modulating the intensity profiles of the circles of confusion in such a way as to become truly Gaussian. Thereby, it is also deemed to improve the "bokeh" of the lens, that is the character of the image in the out-of-focus areas.

The lens offers two separate diaphragms; one nine-bladed circular diaphragm, controlled by the camera when the lens is set to auto mode ("A"), and one ten-bladed perfectly-circular diaphragm, which is controlled by the lens' aperture ring when set to manual settings T4.5 to T6.7.

The F-number in this lens refers to the effective aperture opening and determines the depth-of-field produced by its use. The T-number refers to the amount of light, which passes through the lens and is collected on the film or sensor, as such, transmission stops are used by the camera to calculate exposure. Both values differ significantly, because the tinted glass element remains in the optical path all the time. Fully open, the difference amounts to 1.5 EV, however, the difference will become smaller as the aperture is closed, just as the smoothening effect will become reduced.

The optical effect can be emulated by combining depth-of-field bracketing with multi exposure, as implemented in the Minolta Maxxum 7's STF function.


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