Review by Klaus Schroiff, published May 2012

Special thanks to Dieter Scherk for providing this lens for testing purposes!

Introduction

The Sony 135mm f/2.8 [T/4.5] STF is a unique lens in many ways. The most obvious difference compared to conventional lenses is the lack of AF. So why's that ? The lens has a so-called apodisation optics using one APD element. APD elements resemble neutral density filters which become thicker the more you move towards the perimeter thus reducing the amount of light passing through. Out-of-focus circles are therefore (comparatively) defined in the center but are gradually loosing light towards the edges thus producing a very silky bokeh (out-of-focus blur) - the primary sense of existence of this lens.

This special characteristic has a side-effect though - the "smooth transitions focus" (STF) effect is hostile for AF operations and consequently the focus indicator in the viewfinder remains also non-functional. Obviously an aspect which may produce headaches for some users especially because Sony's "current" full format cameras (A850/A900) don't feature Live-View.

Another consequence of the APD element is a decreased light transmission characteristic (the mentioned ND effect) - you lose about 1 1/2 f-stops (actually 1.5 f-stops) thus the [T/4.5] in the lens' name. The T-value does not change the principal speed (min. depth-of-field) of the lens (=max f/2.8) but you will only see the T-value in the viewfinder.

The build quality of the Sony lens is superb thanks to a metal construction and tight tolerances. The focus ring operates very smoothly and damped just like in the good ol' days.
Unlike typical Sony Alpha lenses, the STF has a dedicated aperture control ring which allows you to set the aperture steplessly between T/4.5 and T/6.7 - this controls a secondary aperture with 10 aperture blades. When selecting the "A" position the primary aperture (9 aperture blades) is controlled by the camera.
You may notice in the product images below the lens extends significantly when focusing at close distances. The front element does not rotate.

If you are interested in further details you may have a look at the following website http://www.the135stf.net/.